Chapter 15. Second and Third Trimester Pregnancy

Over the last 30 years, ultrasound has played an essential role in the care of the obstetric patient. The body of knowledge and expertise in obstetric sonography is robust. Ultrasound is the primary imaging modality for evaluation of uterine, cervical, and amniotic fluid abnormalities; placental and umbilical cord problems; and determination of gestational age, fetal congenital abnormalities, multiple gestation, and fetal presentation.1 While many parts of the examination are not relevant in the emergency setting, there are some findings that may be critical to the acute care of an obstetric patient. This chapter discusses the use of point-of-care ultrasound to evaluate pregnant patients in their second and third trimesters. During this time period, the major indications for its use are in the initial assessment of vaginal bleeding, labor, trauma, and abdominal pain. Emphasis will be placed on a focused or goal-directed ultrasound examination to rapidly measure fetal cardiac activity, estimate gestational age, and exclude placenta previa. Additional applications include assessment of cervical length and fetal position, and for the evaluation of traumatic and nonobstetrical causes of abdominal pain.

In early pregnancy, point-of-care ultrasound is commonly used to help rule out ectopic pregnancy by confirming an intrauterine pregnancy. The clinical indications for point-of-care ultrasound in early pregnancy, the limited information sought, and the recommended technique are widely agreed upon and well described (see Chapter 14, “First Trimester Pregnancy”). In contrast, the role of point-of-care ultrasound in the second and third trimesters of pregnancy is not as well established. Yet, emergency care providers are frequently faced with evaluating patients who are in the latter part of pregnancy and present to the ED with trauma, vaginal bleeding, or abdominal pain. Depending on the practice setting, obstetrical consultation may not be readily available and patients may not have had prenatal care. Increasingly, point-of-care ultrasound is readily available in the emergency setting and clinicians are adept at its use. There are clearly a number of clinical situations during the second and third trimesters of pregnancy where a rapid, goal-directed ultrasound examination can expedite the diagnosis and improve the care of mother and fetus.

A discussion of the use of point-of-care ultrasound in the second and third trimesters of pregnancy must begin by addressing the following questions:

1. What are clinical indications for point-of-care ultrasound in the second and third trimesters of pregnancy?

2. Which focused ultrasound applications are reasonable for acute care providers to perform during late pregnancy?

3. What are the goals of the point-of-care ultrasound exam in the second and third trimesters of pregnancy?

4. Is ultrasound safe, and what are the alternative diagnostic modalities in pregnancy?

What are the clinical indications for point-of-care ultrasound in the second and third trimesters of pregnancy? The concept that point-of-care ultrasound should remain goal-directed helps define its appropriate use in the latter part of pregnancy. In this setting, the focused concept takes on particular importance for the following reasons. First, since the scope and quantity of information potentially available using ultrasound in late pregnancy are enormous, the clinician performing ultrasound in the emergency setting must have a distinct, clinically relevant goal in mind before performing the exam. It would be inappropriate, for example, to assess fetal cardiac morphology in the emergency setting. Second, the medicolegal ramifications of basing clinical decisions on point-of-care ultrasound in the obstetric patient mandate caution. In one analysis of malpractice claims involving diagnostic ultrasound, obstetric ultrasound constituted 75% of the cases.2 Not only should the information sought with each application be carefully limited, but ultrasound should be used only for emergency indications where the immediate benefit of the information outweighs the possibility of a missed diagnosis.

Major clinical indications for the use of point-of-care ultrasound that seem to satisfy these constraints include the initial evaluation of a pregnant trauma patient, mid- and late-trimester vaginal bleeding, labor, and abdominal pain during pregnancy.

Which focused ultrasound applications are reasonable for acute care providers to perform during late pregnancy? Several standard point-of-care ultrasound applications are indicated in late pregnancy and are considered safe and accurate in this setting. These include the focused assessment with sonography for trauma (FAST) examination to assess for significant hemoperitoneum, a right upper quadrant ultrasound examination to assess for gallstones and signs of cholecystitis, and a renal ultrasound examination to evaluate for severe hydronephrosis. Ultrasound may also be used to rapidly visualize fetal cardiac activity. It is also reasonable to estimate gestational age by measuring biparietal diameter (BPD) or femur length. In addition, transabdominal ultrasound can be used to evaluate for possible placental abruption, to exclude placenta previa, and to ascertain the position of the fetus. Transvaginal ultrasound may be used to clarify the relationship of the placenta to the internal os and to measure cervical length.

What are the goals of the point-of-care ultrasound exam in the second and third trimesters of pregnancy? In other words, given the patient's clinical problem, what clinically important question(s) can be answered rapidly with a focused ultrasound examination? For example, in a trauma patient who is comatose and appears to be pregnant, the following questions may be answered with point-of-care ultrasound: Is the patient pregnant? Is there evidence of free intraperitoneal fluid (from hemoperitoneum or uterine rupture)? Is the fetus alive? What is the gestational age of the fetus (and might it survive in the extrauterine environment)? (Figure 15-1) Is there obvious retroplacental hematoma? Figure 15-2 presents a goal-directed approach to point-of-care ultrasound in the second or third trimester of pregnancy based on the patient's clinical problem.

Is ultrasound safe, and what are the alternative diagnostic modalities in pregnancy? The emergence of ultrasound as the main imaging modality in pregnancy is based, in part, on it being considered extremely safe for the fetus. Human organogenesis largely takes place before the 10th week of gestation, the time period when diagnostic ultrasound is often used. The absence of an association between ultrasound and fetal structural anomalies supports its safety in early pregnancy.3 The critical period of brain development occurs during the 14th–22nd week of gestation. The theoretical adverse affect of ultrasound on the fetal brain, due to production of thermal energy and cavitation, has been the topic of several important epidemiologic studies, all of which have found no deleterious effects on cognitive development.4,5 By contrast, the risk to the fetus associated with exposure to ionizing radiation, particularly from abdominal CT, is considered significant.1

For assessment of fetal well-being, alternatives to sonography include the handheld Doppler stethoscope for measurement of fetal heart tones and continuous cardiotocography. Cardiotocography is a form of fetal assessment that simultaneously records fetal heart rate, fetal movements, and uterine contractions to investigate fetal well-being.

In the absence of a reliable menstrual history, a very rough estimate of gestational age can be obtained by measuring fundal height. This crude test represents the only alternative to sonography for estimating gestational age when the history is unclear.

For the evaluation of vaginal bleeding in late pregnancy, sonography has supplanted the traditional double setup and potentially harmful physical examination for excluding the diagnosis of placenta previa. Placental abruption is usually diagnosed using clinical information, by excluding the diagnosis of placenta previa with ultrasound, and with the assistance of cardiotocography. The use of magnetic resonance imaging has been described in the workup of relatively stable patients with unexplained vaginal bleeding, but it is not widely used.6

Ultrasound provides information that is complimentary to the physical examination and cardiotocography in the evaluation and management of preterm or precipitous labor. Ultrasound is the only imaging modality used in this setting to assess cervical length, amniotic fluid volume, and fetal position.

Abdominal pain of unclear etiology is an important clinical indication for use of point-of-care ultrasound in late pregnancy where alternative imaging modalities—such as plain radiography and CT—are commonly employed in nonpregnant patients with similar complaints.

The clinical indications for performing point-of-care ultrasound in the second and third trimesters of pregnancy are as follows:

1. Determination of gestational age and fetal heart rate

2. Assessment of obstetrical causes of abdominal pain and vaginal bleeding

3. Evaluation of labor

4. Evaluation of traumatic injuries

5. Assessment of nonobstetrical causes of abdominal pain

Determination of Gestational Age and Fetal Heart Rate

Fetal assessment begins with measurement of fetal cardiac activity to establish fetal viability. This step is recommended early, not only in evaluation of the pregnant trauma patient7 but also in patients with vaginal bleeding8 and preterm labor.9 The normal fetal heart rate following the first trimester is 120–160 beats per minute (bpm). Sustained bradycardia is often associated with fetal hypoxia and acidemia. Historically, in emergency medicine, fetal cardiac activity was established by measuring fetal heart tones with a handheld Doppler stethoscope.

Point-of-care ultrasound is an attractive alternative to the handheld Doppler stethoscope for initial detection of fetal cardiac activity. Using B-mode scanning of the gravid uterus, locating the fetal heart, and assessing the presence of cardiac motion is relatively straightforward. Using M-mode scanning, the waveform produced by cardiac motion can be recorded. Fetal heart rate is then determined rapidly and accurately with the aid of obstetrics software contained in most ultrasound machines.

Knowledge of gestational age is critical in many clinical settings. This information may be used to guide the decision to perform an emergency cesarean section, such as in the setting of maternal cardiac arrest.10 In addition, gestational age and fetal maturity often influence the management of placenta previa, preterm labor, rupture of membranes, eclampsia, and other severe medical illnesses in late pregnancy. In the setting of trauma, sonographic assessment of gestational age, along with fetal heart rate, can be performed as an adjunct to the initial E-FAST examination. In a stable trauma patient, gestational age may influence the decision to proceed to exploratory laparotomy.11

When assessing gestational age, consider the following general points. By current convention, obstetric dating begins with the first day of the last normal menstrual period, referred to as gestational age or menstrual age, and is equal to the fetal or conceptual age plus 14 days. Assume fetal viability when the gestational age is >24 weeks. The pregnancy is considered “term” at 38 weeks. Sonographic estimates of gestational age are progressively less accurate in later pregnancy due to natural variations in the size of the fetus (Table 15-1). A simple rule that reinforces this principle is that the variability (2 SD from the mean) of a gestational age estimate is equal to approximately 8% of the predicted age.12 An ultrasound measurement of BPD that yields a gestational age of 32 weeks has a variability of ±19 days. Nevertheless, estimates based on BPD, obtained as late as 20 weeks, still outperform menstrual history for predicting onset of labor.13 If possible, base gestational age on results of an ultrasound examination performed prior to 20 weeks or on a reliable menstrual history. Beyond 20 weeks, point-of-care ultrasound may be used to estimate gestational age if menstrual dates are unreliable or when the patient cannot provide a history.

In the first trimester, crown-rump length is the preferred biometric measurement for establishing gestational age. In the second and third trimesters, measurements commonly used to estimate gestational age include BPD, head circumference (HC), and femur length. Modern ultrasound machines contain software that will automatically calculate gestational age based on any one of these parameters. In choosing which biometric parameter to measure, the established predictive validity of the parameter (Table 15-1) should be considered, and the ease and speed with which it can be obtained. A parameter that has excellent predictive validity according to the obstetric literature, but is difficult to measure and therefore prone to error, may not be well suited to the emergency setting.

In the second trimester, BPD and HC are the most widely used measurements to determine gestational age.12,14 Although in expert hands, HC has a somewhat better predictive validity than BPD, measurement of BPD is preferable in the emergency setting because of the relative ease with which it is obtained. While HC must be calculated in a particular plane, BPD can be measured in any plane, provided the line of measurement intersects the thalamus and third ventricle.12,15 Of particular relevance to emergency medicine providers is a study that directly correlated neonatal survival in premature infants with various biometric measurements obtained by ultrasound shortly before birth. Based on analysis of receiver operator curves, a BPD of >54 mm was the single best predictor of survival.16

In the third trimester, femur length is a frequently used alternative to BPD for estimating gestational age.12,17 In late pregnancy, measurement of BPD may be difficult because the fetal skull is frequently located within the maternal pelvis and can be obscured by acoustic shadowing. The predictive validity of femur length is slightly better than BPD at this stage.14 Femur length is relatively easy to measure because the transducer need only be parallel to the long axis of the femur.12,18

In the late third trimester, identification of an ossified distal femoral epiphysis represents a potentially rapid means of estimating gestational age, and may be useful in the emergency setting. The appearance of this ossification center indicates a gestational age of 29 weeks or greater, whereas its absence means that the gestational age is <34 weeks.19 Similarly, the appearance of an ossified proximal tibial epiphysis suggests a gestational age of at least 35 weeks indicating the fetus is at or very near term.12

Assessment of Obstetrical Causes of Abdominal Pain and Vaginal Bleeding

The primary pregnancy-related causes of abdominal pain in the second and third trimesters are preterm labor, placental abruption, and chorioamnionitis.20–24 Chorioamnionitis refers to an infection of the amniotic fluid, typically following a rupture of membranes, or rarely as a complication of diagnostic amniocentesis. In rare cases, infection may occur without membrane rupture, causing pain, preterm labor, and systemic signs of infection. The diagnosis can be made by ultrasound-guided amniocentesis.25

There are several disorders unique to pregnancy that may cause abdominal pain. Severe pregnancy-induced hypertension can be complicated by HELLP syndrome in 5–10% of cases, which is characterized by hemolysis, elevations in liver function tests, and low platelets. Midepigastric or right upper quadrant pain is present in 25% of cases.25 Spontaneous liver or spleen subcapsular hematomas can also develop, usually, but not always, in association with pregnancy-induced hypertension. Patients may experience right or left upper quadrant pain and mild coagulopathy, but have normal liver function tests. Hematoma rupture results in peritonitis and hemorrhagic shock.24 The diagnosis is often difficult, since this condition may resemble uterine rupture or abruption, and ultrasound may be helpful in differentiating these conditions.

Patients with vaginal bleeding in the second and third trimesters are at high risk, with fetal mortality or adverse outcomes in up to one-third of all cases.25,26,27 Vaginal bleeding beyond 20 weeks' gestational age complicates 5% of all pregnancies. In 13% of those with bleeding, placental abruption is the cause, and in 7% placenta previa is the cause.28,29 Placenta previa and abruption account for the vast majority of cases requiring transfusion or cesarean section, as opposed to cases of vaginal bleeding caused by early labor, lower genital lesions, or other diagnoses.

Placenta Previa

The term placenta previa refers to a placenta that completely covers the internal cervical os. Placenta previa has traditionally been subdivided into complete, meaning that the entire os is covered by placenta, and partial, meaning the os is partially covered. When the placental edge is located within 3 cm of the internal os, it is termed marginal placenta previa. The term low-lying placenta is useful for describing the case of a placenta located in the lower portion of the uterus in which the exact os–placenta relationship cannot be defined, or for describing an apparent placenta previa when seen in the second trimester (Figure 15-3).

Placenta previa is present at term in only approximately 0.5% of pregnancies. Yet, routine ultrasound in early second trimester has found low-lying placenta in up to 45% of patients and an apparent placenta previa in 5%.30–32 The explanation for this paradox is widely referred to as placental migration, the relatively rapid elongation of the lower uterine segment during the third trimester, which effectively moves the placenta away from the os.33

Maternal risk factors for placenta previa include advanced age, multiparity, non-Caucasian race, previous cesarean section, and prior history of placenta previa.33,34 Placenta previa usually presents as painless vaginal bleeding. However, pain from contractions sometimes accompanies the hemorrhage. The first episode of bleeding typically occurs in the third trimester, but may not occur until after the 36th week in up to one-third of cases.33

The evaluation of possible placenta previa begins with transabdominal scanning as it is rapid, noninvasive, and reliable for locating the placenta (Figure 15-4) A digital vaginal examination can precipitate severe hemorrhage in the presence of placenta previa. Ultrasound can be used to locate the placenta and exclude placenta previa prior to vaginal examination. The sensitivity for ultrasound diagnosing placenta previa is 92–98%.35,36 When the placenta is visualized at or near the fundus by transabdominal ultrasound, placenta previa is effectively excluded. After excluding the diagnosis of placenta previa, the clinician can then proceed to evaluate the patient for placental abruption. However, if the placenta is clearly seen covering the entire cervical os, particularly in the third trimester, the diagnosis of placenta previa is confirmed. When the placenta appears to be low lying or partially covering the os, or when an adequate view cannot be obtained with transabdominal ultrasound, further evaluation with transvaginal or translabial ultrasound is generally indicated. The diagnosis of placenta previa by transabdominal ultrasound has a high false-positive rate, up to 17% in one large study.37 Placenta previa is often overdiagnosed in the second trimester because it is mimicked by two conditions: (1) an overdistended bladder that compresses the lower uterine segments and (2) focal contractions.38 With the transabdominal approach, the relationship of the inferior edge of the cervix to the internal os is frequently obscured by patient obesity, an overdistended bladder, myometrial contractions, a posterior placenta, or the ossified fetal skull.39,40 In one study of patients with suspected previa, assessment of the placenta–os relationship was impossible in 31% of transabdominal studies.41 Nevertheless, in the case of severe hemorrhage, if findings on transabdominal ultrasound appear to be consistent with placenta previa, the patient should proceed directly to the operating room. A double setup examination may then be performed in the operating room at the obstetrician's discretion.

The approach to management of placenta previa depends largely on the sonographic assessment of fetal well-being and gestational age. Although cesarean section is the definitive treatment, vaginal bleeding due to confirmed placenta previa is frequently managed in an expectant fashion. The rationales for expectant management are as follows: (1) bleeding prior to the third trimester is often self-limited and can be treated by transfusion, if necessary; (2) vaginal bleeding represents little direct risk to the fetus in the absence of significant abruption or maternal shock; and (3) delaying delivery, to maximize fetal maturity, improves perinatal outcome.33 Confirmation of fetal well-being is a prerequisite to expectant management. A rapid initial measurement of fetal cardiac activity can be performed in the emergency setting with transabdominal ultrasound, although cardiotocography is then required for ongoing fetal monitoring. Other measures of fetal well-being, such as amniotic fluid volume and biophysical profile, may have an impact on management.

A determination of gestational age also is required to guide treatment decisions.8 The following general guidelines have been proposed with regard to management of previa based on gestational age. When gestational age is <24 weeks, delivery is indicated only for hemorrhage that is life threatening to the mother. Between 24 and 34 weeks, fetal distress and life-threatening hemorrhage are indications for delivery. Beyond 34–37 weeks, delivery is indicated for fetal distress, significant bleeding, or labor.33

Placental Abruption

Any abnormal separation of the placenta occurring after 20 weeks' gestation is defined as a placental abruption. Prior to this date, placental separation is considered part of the process of a spontaneous abortion. While it affects <1% of all pregnancies, abruption accounts for more than a quarter of all perinatal mortality.8,33 The epidemiology of placental abruption suggests that a variety of risk factors contribute to its development, many of which relate to more general microvascular disease. One of the strongest associations is with maternal hypertension, both chronic and pregnancy induced.42 Cigarette smoking and cocaine abuse have also been linked to higher rates of abruption.43,44 Trauma is an uncommon but important cause of abruption. Particular attention must be given to victims of motor vehicle crashes, falls from height, and domestic violence.28,33,45

Hemorrhage from abruption begins at the point of separation between the placenta and the uterus, or the placenta and the amnion. The timing and degree of subsequent bleeding from the cervix are dependent on the size of the hemorrhage and its location relative to the placenta. The amount of vaginal bleeding is not a reliable guide to the degree of placental abruption or the severity of hemorrhage. In some cases, patients may experience no vaginal bleeding despite significant placental separation. The amount of vaginal bleeding must never be taken as a guide to degree of internal hemorrhage. Also, the presence of abdominal pain, considered a hallmark symptom for abruption, is absent in nearly half of all cases.8,33 Therefore, when evaluating patients with painless vaginal bleeding in pregnancy, the possibility of abruption must be considered.7,33 The most consistent finding in abruption will be the presence of uterine irritability and contractions.7 These may be unappreciated by both patient and physician without the aid of cardiotocography.

Because neither the character of the bleeding nor the presence of pain can be relied upon to differentiate placental abruption from placenta previa, it is recommended that the evaluation of vaginal bleeding in the second and third trimesters begins with an ultrasound examination to exclude placenta previa. Once the diagnosis of placenta previa is excluded, abruption becomes the major diagnostic consideration.

During placental abruption, hemorrhage occurs within a layer of the placenta causing separation from the adjacent uterine wall.33 This hemorrhage and separation may remain partial and self-limited or progress on to complete abruption. The clinical manifestations of abruption and its prognosis are directly determined by the extent to which placental circulation is compromised, which in turn depends on both its size and location. Hemorrhage confined to the edge of the placenta is referred to as a marginal abruption. The degree or size of placental-uterine separation is graded as mild (grade 1), partial (grade 2), or complete (grade 3) (Figure 15-5).27,46 These categories correlate reasonably well with the clinical presentation and prognosis. Grade 1 separations are usually marginal, involve less than a few centimeters of the placental border, and are usually not clinically significant. In contrast, grade 3 abruption can be fatal to both the fetus and the mother. Abruption is further categorized by the anatomic location of the hematoma relative to the placenta: retroplacental (in the decidua basalis, between placenta and uterine wall), subchorionic (between decidua and the membranes), and preplacental (between placenta and amniotic fluid, immediately beneath the amnionic membrane) (Figure 15-6).47 These distinctions also have significant prognostic implications, but primarily for fetal outcome. A retroplacental hemorrhage of 60 mL or more results in a 50% fetal mortality rate, but a similar-sized subchorionic hemorrhage results in a 10% fetal mortality rate. Preplacental hemorrhage is often self-limited and clinically silent, with 30% detected only after delivery.27,47 They appear sonographically as an irregular bulge along the inner border of the placenta. Rupture of these hematomas results in the classic “port-wine” staining of the amniotic fluid.

Ultrasound is not a sensitive test for placental abruption and the diagnosis remains largely clinical, beginning with a meticulous search for evidence of vaginal bleeding, uterine tenderness, labor, or fetal distress. Signs of labor and fetal distress are the keys to making the diagnosis. Uterine contractions are present in nearly all cases of abruption, although they may be difficult to appreciate by either the physician or the patient. Contractions are characteristically of high frequency but low amplitude. It has been demonstrated that 6 hours of cardiotocographic monitoring after trauma is 100% sensitive for predicting all subsequent complications.7 Even rare cases of late-onset abruption or fetal distress after trauma are heralded by early abnormalities on cardiotocography.48 Hence, the absence of uterine irritability or fetal distress remains an excellent indicator of maternal–placental well-being, suggesting that abruption is either absent or clinically insignificant. Cardiotocography should be a routine part of the initial evaluation where there is a concern for abruption, even in those who are asymptomatic. The stable patient without evidence of uterine irritability for 6 hours may be discharged home with appropriate instructions and follow-up.

Once placental abruption is diagnosed, an immediate cesarean section remains the definitive treatment. Decisions regarding the manner and timing of any intervention depend on an overall assessment of maternal–fetal well-being. If the fetus is immature and the abruption is judged to be mild, an expectant approach may be attempted. Signs of preterm labor may be difficult to distinguish from mild abruption.33 A term fetus or evidence of uterine irritability refractory to medical management should prompt expedited delivery. Similarly, fetal distress or maternal signs of abruption indicate a need for immediate cesarean section.25,33

Evaluation of Labor

Approximately 7% of newborns are premature at birth, which may result in mental or physical impairment.49 Preterm labor is defined as regular uterine contractions accompanied by characteristic changes in the cervix, occurring prior to 37 weeks' gestation. Assessing the potential for premature delivery is the main goal of the emergency evaluation for preterm labor.

Ultrasound is a safe, rapid, and accurate method of evaluating the cervix for signs of labor. As labor begins, the cervix undergoes effacement followed by dilation. While the digital examination has traditionally been used to evaluate such cervical changes, ultrasound has emerged as a safer and more accurate means to evaluate cervical changes.50 Sonographic measurement of cervical length represents an objective way to quantify effacement.

There are several justifications for using an ultrasound examination rather than the digital examination in this setting. First, a direct contraindication to digital examination may exist, such as placenta previa or ruptured membranes, and digital examination in these settings can produce life-threatening bleeding or chorioamnionitis. A transabdominal or translabial ultrasound examination may negate the need for a digital examination. Second, an ultrasound examination has been shown in numerous studies to be more accurate than digital examination in estimating cervical length and predicting preterm labor.50

An advantage of using ultrasound is that it can visualize the internal cervical os and detect funneling. Funneling is dilatation of the internal cervical os without dilation of the external cervical os, and it is one of the earliest signs of labor. A digital examination cannot palpate the internal os, so it cannot detect funneling. An ultrasound examination is the only practical way to detect this important finding.50–55

There are three methods for imaging the cervix with ultrasound: transabdominal, transvaginal, and translabial. The transabdominal approach is the least reliable method, with successful imaging of the cervix in only 46% of patients without a full bladder and 86% with a full bladder.56 Presenting fetal parts and a large maternal habitus may obscure visualization of the cervix. The transvaginal technique produces the most consistent findings, with visualization of the cervix in up to 100% of patients.51 The translabial ultrasound is considered the most technically difficult method. In the hands of a skilled operator, however, this approach provides an adequate view of the cervix in up to 95% of patients.51,57 In the emergency setting, it is reasonable to begin with transabdominal ultrasound and then proceed to transvaginal or translabial imaging if the cervix cannot be well visualized.

From studies of all three ultrasound methods, it is clear that cervical measurements can predict the risk of preterm delivery. In one study of nearly 3000 patients at 24–28 weeks' gestation, it was demonstrated that a positive correlation exists between short cervical length (<30 mm) and risk for preterm birth before 35 weeks (Figure 15-7).53 Another study of patients between 16 and 28 weeks' gestation found a 79% rate of preterm delivery in those with cervical funneling of >50%.58 Subsequent studies have confirmed the predictive value of both short cervical length and the presence of funneling, and have extended the findings to twin pregnancies.55,59 For the most part, patients in these studies were asymptomatic so applicability in the emergency setting is unclear. Also, the impact of such findings on clinical management, such as the need for cervical cerclage, remains uncertain.

The use of ultrasound in determining the presence or absence of cervical changes can help the emergency physician risk stratify patients with symptoms suggestive of labor. The goal of such an evaluation is to identify patients who would benefit from admission and tocolysis versus candidates for outpatient follow-up. The utility of cervical sonography in the management of preterm labor was retrospectively evaluated at one facility. Women were hospitalized only if they had a cervical length <30 mm. This protocol produced a decrease in hospital days of 48% without affecting the rate of preterm births.60 The critical cervical length appears to be 30 mm, with preterm delivery much more likely in patients with a shorter cervical measurement by ultrasound.60 Apply cervical sonography in conjunction with clinical variables—such as the results of a biophysical profile (discussed below), the gestational age, and a history of preterm births—to influence management decisions.

Lie refers to the relationship of the fetus to the long axis of the uterus, while presentation describes what fetal part is nearest the cervix. In normal deliveries, the fetal lie is longitudinal and the presentation is cephalic. Transverse lie and breech presentation—where the fetal sacrum and feet, respectively, are engaged in the pelvis—are referred to as malpresentations. The classification of breech presentations is demonstrated in Figure 15-8.

In the emergency setting, it is absolutely essential to know the presenting fetal part. Even with good prenatal care, fetal presentation is often unknown because prior to 25 weeks the fetus frequently changes position. Breech presentations, which account for 3–4% of all deliveries, are fraught with complications such as asphyxia, cord prolapse, and spinal cord injuries to the fetus.61 Knowledge of the presentation allows the emergency physician to mobilize the appropriate equipment and support staff needed for delivery. In an emergency vaginal delivery, the presenting part is discovered easily by physical examination of the vagina or perineum. Prior to this point, determining fetal position by palpation may be difficult, particularly for a nonobstetrician. Moreover, vaginal examination is contraindicated when preterm labor is accompanied by vaginal bleeding. In such cases, an ultrasound examination can be used to establish fetal position.

Just as important as fetal position is the determination of the number of fetuses prior to delivery. Ultrasound is invaluable for identifying the “surprise twin.” Perinatal death occurs seven times more frequently in twin deliveries compared to singletons.62 Like breech presentations, delivery of twins requires additional expertise, support, and equipment.

There are multiple noninvasive measures to evaluate the health of the unborn child. Traditionally, nonstress testing and amniotic fluid volume measurement have been used. The biophysical profile is a more sophisticated instrument that combines nonstress testing and amniotic fluid volume with three additional sonographic parameters—fetal tone, movement, and breathing—to derive an objective score that reflects overall fetal well-being (Table 15-2).63–65 The idea behind the biophysical profile is that the CNS, which is very sensitive to hypoxia, controls all of the measured parameters. Thus, a low biophysical profile score may indicate either acute or chronic fetal hypoxia.66,67 Results of the biophysical profile are always considered together with gestational age and maternal and fetal comorbidities, but in general, lower scores have increased risk of fetal hypoxia, and scores >8 are considered normal.65–72

Evaluation of Traumatic Injuries

Trauma occurs in 6% of pregnant patients73 and is the leading cause of nonobstetrical maternal mortality.74 Furthermore, fetal loss as a result of trauma far exceeds maternal mortality.75 Point-of-care ultrasound has the potential to play a critical role in the initial evaluation of pregnant trauma patients. Use of the E-FAST examination to evaluate maternal intra-abdominal hemorrhage has been shown to be beneficial.11 The sensitivity and specificity of the E-FAST examination in pregnant trauma patients are similar to the sensitivity and specificity in nonpregnant trauma patients.76 Because fetal well-being is dependent on adequate maternal circulation, a tenant of trauma care is that resuscitation of the mother and assessment of maternal injuries is the initial priority.

Besides maternal shock, processes that contribute to fetal loss include direct fetal injury, uterine rupture, placental abruption, ruptured membranes, and premature labor. Factors associated with fetal loss after maternal trauma are listed in Table 15-3.77–82

Placental abruption is the most common cause of fetal loss in pregnant trauma patients, and may occur after relatively minor trauma and in the absence of other injuries.7 While cardiotocography and observation for the occurrence of frequent contractions remain the cornerstone of the evaluation,73 ultrasound may sometimes be able to confirm the presence of significant abruption.7 In addition, point-of-care ultrasound may be used to rapidly demonstrate fetal cardiac activity and determine gestational age.11

Perform the E-FAST examination either immediately following or, preferably, contemporaneously with the primary survey of the pregnant trauma victim. The sonographic evaluation of pregnant trauma victim is a combination of the E-FAST exam as well as three other rapid and goal-directed scanning protocols. In order to perform this combination of ultrasound studies, the preferred transducer is the curvilinear, due to its lower frequency range and obstetric preset calculations. First and foremost is the sonographic assessment of potential maternal life threats with the E-FAST examination. This will allow the emergency provider to rapidly determine the presence of intraperitoneal fluid, pericardial fluid, pleural fluid, or pneumothorax (see Chapter 5, “Trauma”). Following the E-FAST exam, change the machine settings to OB and answer four questions: (1) Does the patient have a sonographically visible intrauterine pregnancy?; (2) What is the gestational age?; (3) Is there fetal cardiac activity?; and (4) What is the fetal heart rate?

Assessment of Nonobstetrical Causes of Abdominal Pain

Abdominal pain in the second and third trimesters of pregnancy represents a significant challenge for clinicians. The differential diagnosis must include obstetrical and nonobstetrical causes of abdominal pain, and the impact of diagnostic and management decisions on both the mother and the fetus must be considered. The anatomic and physiologic changes of pregnancy may alter the traditional presentation of many disorders, and the symptomatology of pregnancy itself may overlap with that of nonobstetrical abdominal pathology. Concerns over ionizing radiation or medication-induced teratogenicity may limit choices of diagnostic imaging or therapy. All of these elements may confuse the clinical picture and delay definitive diagnosis, increasing the risk of morbidity to the mother and the fetus. Ultrasound offers a safe and effective first step for the evaluation of abdominal pain in pregnancy.

Nonobstetrical causes of abdominal pain in pregnancy occur at rates similar to that of the nonpregnant population. Similarly, the need for urgent abdominal surgery during pregnancy parallels that of the general population when ectopic pregnancies and cesarean sections are excluded.20,21 However, the morbidity associated with virtually every abdominal emergency is higher in pregnancy, mostly due to the difficulty in making an early diagnosis.22

The evaluation of abdominal pain in the second and third trimesters demands that clinicians maintain a broad differential and an awareness of the altered presentation of common abdominal disorders in pregnancy. Table 15-4 lists the common obstetrical and nonobstetrical causes of abdominal pain. Every workup begins with a careful history and physical examination that should significantly narrow the differential. Laboratory tests must be interpreted in the context of pregnancy-induced changes (e.g., leukocytosis and relative anemia) and are frequently of limited diagnostic value.

Sonography is a reasonable first diagnostic step in most cases and helps to substantially narrow the differential diagnosis, if not confirm the diagnosis. As an example, right upper quadrant pain in the third trimester engenders a broad differential that should include biliary disease, liver disease, nephrolithiasis, pyelonephritis, and appendicitis. Normal laboratory tests and urinalysis may help to reduce the list of likely possibilities to biliary colic and appendicitis. Ultrasound could then be used to confirm or exclude both of these disorders while providing additional information on the pregnancy itself.

Ultrasound is the traditional first-line imaging modality for abdominal pain in pregnancy because it provides anatomic and functional information about the mother and fetus without exposing either to the effects of ionizing radiation. Intrauterine exposure to radiation may have both oncologic and teratogenic effects. Case-controlled studies of childhood cancer show a slight but significant increase in relative risk among the children of female radiologists exposed to 1000 mrems of radiation.21 The potential for teratogenicity and carcinogenesis appears to be greatest in the period of 2–15 weeks and decreases proportionally as the fetus nears term. In the first trimester, in utero radiation exposure is graded as low, moderate, or high depending on the total dose, with the clearest evidence of harm above a threshold level of 150 mGy (Table 15-5).23 Significant variability exists, however, depending on gestational age, body habitus, and the type of radiographic study. Exposure in the second and third trimesters is less critical with an estimated relative increase in cancer risk of 64% per rad (10 mGy), which corresponds to a 0.05% relative increase in the rate of childhood malignancies. Fortunately, the majority of radiographic tests fall well below the threshold level (150 mGy or 15 rads) and plain radiography and CT remain potential diagnostic tools in pregnant patients with abdominal pain.23 That said, the decision to use radiography in a pregnant patient must take into account a number of competing elements: the risk of radiation to the developing fetus, the risk of delayed diagnosis if available imaging techniques are not used, and the relative suitability of alternative imaging or management strategies. There are also the intangible concerns about radiation exposure that may not be supported by data, but nevertheless push providers toward using alternative imaging modalities. The use of diagnostic radiography in the pregnant patient is appropriate provided the risks and benefits are weighed in a manner that ensures the best outcome for the mother and fetus. Regardless of the relative risks and benefits of radiography, ultrasound is a valuable initial tool in the evaluation of the pregnant patient with abdominal pain, given its safety and unique ability to assess fetal well-being.

As pregnancy advances into the second and third trimesters, a number of anatomic and physiologic changes occur. These changes alter the way a variety of common disease processes present. The primary sonographic approach to these disorders must change accordingly. As the uterus enlarges, the intestinal tract is displaced upward, backward, and to the sides. The appendix moves to the right upper quadrant and away from the omentum (Figure 15-9). Increased intra-abdominal pressure leads to increased gastroesophageal reflux, and the gravid uterus compresses the ureters, inferior vena cava, and bladder.20,21 Pregnant women commonly experience varying degrees of anorexia, nausea, vomiting, and back and flank pain, which are all related to the compressive and postural effects of the gravid uterus. Serious intra-abdominal processes can cause similar symptoms.

Appendicitis

Appendicitis is the most common surgical emergency in pregnancy, accounting for two-thirds of all laparotomies.25,83 In the first half of pregnancy, the clinical presentation of appendicitis is similar to that in nonpregnant patients. Thereafter, the clinical picture becomes more atypical.25 As mentioned above, early constitutional symptoms of appendicitis are often subtle and dismissed as normal symptoms of pregnancy. The abdominal pain is predominantly right-sided and corresponds to the location of the appendix in different stages of pregnancy (Figure 15-9). Leukocytosis is a common but unreliable finding, and 20% of pregnant patients with appendicitis have sterile pyuria.84 The differential diagnosis includes ovarian mass, ovarian torsion, and other sources of right upper quadrant pain, specifically cholecystitis, pyelonephritis, or hepatitis.20,21,85

If appendicitis is suspected on clinical grounds, the diagnosis may be confirmed by graded compression ultrasound, which is highly accurate for the diagnosis of appendicitis during pregnancy. In one study of 45 pregnant women with abdominal pain, graded compression ultrasound demonstrated a sensitivity of 100% and specificity of 96% for the diagnosis of appendicitis.86 However, earlier studies found a considerably lower sensitivity of 75–89%, although with similar specificities.87,88 These results, along with its well-established diagnostic performance in nonpregnant patients, would suggest that ultrasound for appendicitis in pregnancy should be regarded as a diagnostic test that is specific, but may have limited sensitivity. A positive finding would allow surgical management without the need for other diagnostic testing, but a negative ultrasound exam, in the setting of an intermediate or high pretest probability, would require further testing.

The sonographic evaluation for appendicitis is challenging and highly operator dependent. During pregnancy, this assessment is further complicated by the upward and outward displacement of the appendix by the growing uterus. A graded compression test is performed with special attention paid to the overall diameter of the appendix, wall thickening, or the presence of surrounding fluid and debris. The specific sonographic findings of appendicitis are discussed in Chapter 11, “General Surgery Applications.” In the third trimester, the size of the uterus may preclude adequate visualization of the appendix, despite a proper high-lateral approach.86 Placing the patient in the left lateral decubitus position may make visualization of the appendix more likely.

Laparoscopy or laparotomy, without any diagnostic testing, may be indicated. It has been recommended that surgeons maintain a lower threshold for surgical exploration in pregnant patients with possible appendicitis, given the variability of clinical signs and the increased morbidity associated with diagnostic delay. Negative exploration rates as high as 40% in the third trimester are commonplace and may be acceptable.88

The functional absence of the omentum in pregnancy means that a ruptured appendix is less likely to be walled off, resulting in early peritonitis. Perinatal mortality rises from 4.8% in nonperforated appendicitis to 27.8% when the appendix ruptures. The perforation rate prior to diagnosis is as high as 30% during the third trimester.83 Since the risk of fetal loss and maternal morbidity from a delayed diagnosis of appendicitis is considerable, clinical vigilance is required even in the absence of classic signs and symptoms.21

Cholelithiasis and Cholecystitis

Acute gallbladder disease is more common in pregnancy than in the nongravid population, which reflects a higher prevalence of gallstones in fertile women (3.5–11%).89,90 Signs and symptoms are essentially the same as in the general population: abrupt onset of stabbing or colicky right upper quadrant abdominal pain accompanied by nausea and vomiting. While the presence of fever and Murphy's sign suggests acute cholecystitis, mild elevations in the WBC, amylase, and alkaline phosphatase can be normal during pregnancy. The differential diagnosis includes appendicitis, pyelonephritis, nephrolithiasis, and rare entities such as the HELLP syndrome or subcapsular hematoma.

Pregnant patients suspected of having biliary disease should invariably have a right upper quadrant ultrasound exam (see Chapter 10, “Hepatobiliary”; Videos 10-1 and 10-2). As in the nonpregnant population, sonography is the imaging modality of choice for biliary disease, and its technique and diagnostic performance are essentially unaltered by pregnancy. Oral cholecystograms and HIDA scans are effective but less attractive options in the pregnant patient due to the risks of radiation exposure. Ultrasound will identify nearly all gallstones when the exam is performed carefully.90 The presence of gallbladder wall thickening or pericholecystic fluid are strong indicators of gallbladder inflammation. Many patients with acute cholecystitis during pregnancy can be managed conservatively with IV hydration, analgesia, and antibiotics. The risk of fetal loss with cholecystectomy approaches 5%, but appears to be lowest when performed in the second trimester. Surgical intervention should not be delayed if the patient becomes toxic or develops pancreatitis, since the fetal loss rate may reach 50% in these patients.90

By the 13th week of gestation, or the beginning of the second trimester, the fundus of the gravid uterus is easily palpable above the pelvic brim and, by 20 weeks, it normally reaches the level of the umbilicus. The distance in centimeters from the pubic symphysis to the fundus approximates gestational age in weeks (Figure 15-10). As the uterus and developing fetus grow, the intestines are displaced posteriorly, superiorly, and toward the flanks. As a result, by the third trimester, the appendix is normally found in the right upper quadrant (Figure 15-9). The growing uterus also compresses the ureters, frequently resulting in asymptomatic hydronephrosis, which occurs more often on the right side.

Familiarity with the anatomy of the lower uterine segment, the cervix, and surrounding pelvic structures is essential for sonographic evaluation of cervical length and placenta previa (Figure 15-11). The normal cervix is 3–5 cm long and cylindrical in shape. The long axis of the cervix normally lies at a right angle to the long axis of the vagina. In late pregnancy, the inner one-third of the cervix, or isthmus, elongates to form the lower uterine segment. The passageway between the uterine cavity and the external os is referred to as the endocervical canal. An area of glandular tissue, which may be hypoechoic or hyperechoic, surrounds the endocervical canal. This glandular zone disappears after 31 weeks' gestation, indicating cervical ripening, and making the canal difficult to locate sonographically.51,91 The bladder lies anterior to the vagina and cervix. As the bladder distends, it impinges on the anterior wall of the lower uterine segment. The rectum and sacral promontory lie posterior to the cervix and lower uterus.

Understanding the sonographic appearance of placental pathology requires familiarity with the underlying uterine and placental anatomy. The placenta is primarily a fetal organ, and its size, thickness, and texture reflect the health and gestational age of the developing fetus. In the early part of the second trimester, the placenta is easily visible sonographically as a homogeneous, hyperechoic rim of tissue adjacent to the gestational sac (Figure 15-12).35 The thickness of the placenta in millimeters approximates the gestational age in weeks and rarely exceeds 4 cm.92

By the latter part of the second trimester, the relevant anatomic segments of the placenta become more established. These include (from outside in) the myometrium, decidua basalis (endometrial–placental interface), intervillous space (area of maternal–fetal exchange), chorion (membrane that envelopes the fetal vessels), and amnion (membrane that overlies the placenta, separating it from amniotic fluid) (Figure 15-6). Fetal (umbilical) vessels course within the chorion, whereas maternal (endometrial) vessels are located within the decidua.

The sonographic appearance of these layers is variable. Prominent endometrial arteries and veins within the decidua may appear as hypoechoic bands of tissue separating the myometrium from the fetal placenta, which is termed the retroplacental hypoechoic complex.35 In the intervillous space, sonolucent pools of maternal blood, called villous lakes, can lend a heterogeneous appearance to this middle layer. The addition of Doppler in both of these regions may help determine the vascular nature of a sonolucent area. Lastly, the subchorionic segment may undergo cystic changes associated with fibrin deposition in up to 20% of patients. These echogenic, cyst-like lesions can grow to 1 cm in size but are of no clinical significance.35,92 The normal occurrence of these lesions limits the specificity of the ultrasound in the evaluation of possible abruption.

A discussion of fetal anatomy and development is beyond the scope of this chapter and has limited relevance to point-of-care ultrasound. Fetal anatomic landmarks that must be recognized for the BPD and femur length measurements are described in the “Technique and Normal Ultrasound Findings” section.

Most modern ultrasound machines are capable of imaging for the entire range of pregnancy-related complaints. It is recommended that both abdominal (3.5–5 MHz) and transvaginal (5.0–8.0 MHz) transducers be available. Ultrasound gel (preferably warmed) is a requirement for both scanning techniques. For abdominal scanning, position patients in either the supine or in the left lateral decubitus position. For transvaginal scanning, a dedicated pelvic exam bed (with foot holders and breakaway bottom) will allow for optimal scanning conditions. Sterile setup and transducer covers are required. For transvaginal scanning, place ultrasound gel on the transducer's scanning surface and then cover with a sterile cover. Place sterile surgical lubricant on the transducer cover over the scanning area of the transducer.

Fetal Heart Rate Determination

Determine fetal heart rate by transabdominal scanning using a 3.5–5 MHz transducer. Set the ultrasound machine for simultaneous B-mode and M-mode recording and obstetrical measurements. The specific scanning plane is unimportant, although fanning the transducer in a plane transverse to the fetal spine is a good method of rapidly locating the heart. Position the M-mode sampling line through the heart. Freeze the image when a continuous waveform is evident on the M-mode tracing at a depth corresponding to cardiac movement. Measure the distance between one- or two-cycle lengths (depending on the ultrasound machine's software); the computer will calculate and display the fetal heart rate (Figure 15-13). Avoid using pulsed-wave, continuous-wave, or color Doppler, particularly in the first trimester, because these modalities transmit higher energy.

Estimation of Gestational Age

Biparietal Diameter

Because of its predictive validity and relative ease of measurement, BPD is the biometric measurement of choice for estimating gestational age after 14 weeks. Perform determinations of gestational age during the second and third trimesters by transabdominal scanning using a 3.5–5 MHz transducer. Measure the BPD at the level of the third ventricle, paired thalami, quadrigeminal cistern, and cavum septi pellucidi.14,17 Although it is counterintuitive, the fetal ventricles and subarachnoid space are echogenic compared to brain due to the relative prominence of the choroid plexus and pia-arachnoid matter. The junction of the hyperechoic third ventricle and quadrigeminal cistern forms an easily recognizable sonographic landmark, referred to as the arrow sign (Figure 15-14). It has been proposed that the BPD can be measured in any plane as long as the line of measurement crosses the thalami and the third ventricle.15 Points of measurement should be from the outer edge of the near calvarial wall to the inner edge of the far wall. The near and far calvarial walls should appear symmetric. Be careful not to include overlying soft tissue and scalp in the measurement (Figure 15-15).15

Femur Length

Beyond 26–32 weeks' gestation femur length is an alternative to BPD, and may be easier to measure. Femur length is actually a measurement of the ossified portion of the diaphysis and metaphysis only, and does not include the cartilaginous portions of the bone. Points of measurement are the junction of the ossified metaphysis and the cartilaginous diaphysis at either end of the femur. The distal femoral epiphysis, a secondary ossification center, is not included. Femur length can be measured in any plane as long as it is parallel to the long axis of the bone and includes the accepted end points. To avoid an oblique, falsely short measurement, the plane of section should include the femoral head or greater trochanter and the distal femoral condyle, all of which are cartilaginous (Figure 15-16).18 Further information about gestational age can be obtained by noting whether the distal femoral and proximal tibial epiphyses are ossified (Figure 15-17).

Location of the Placenta

Transabdominal Approach

Begin the sonographic evaluation of vaginal bleeding in the second and third trimester by determining the location of the placenta. Begin with transabdominal scanning using a 3.5–5.0 MHz transducer. Locate the placenta and scan in the sagittal plane to determine whether it extends into the lower uterine segment (Figure 15-18). If so, perform transverse and oblique scanning to determine whether it is centrally or laterally located. When the placenta is low lying, measurement of the os–placenta distance by transabdominal ultrasound is often difficult because the endocervical canal and inferior placental edge may not be visible in the same sagittal plane.33

The bladder should be full for optimal visualization of the lower uterine segment and internal os. However, if the placenta appears to be close to the internal os when the bladder is full, repeat scanning after the patient has voided. A full bladder may create a false impression of placenta previa by pushing the anterior wall of the lower uterus against the posterior wall, artificially elongating the cervix and shortening the distance between the placenta and the internal os (Figure 15-19). Myometrial contractions, which in the second trimester may not be felt by the mother, may also result in a false-positive diagnosis of placenta previa. A myometrial thickness >2 cm is suggestive of a contraction.35 Repeat scanning in 20–30 minutes has been suggested to avoid a false-positive diagnosis of placenta previa due to contractions; however, this option may be unrealistic in the emergency setting.

Transvaginal Approach

Use transvaginal or translabial sonography to clarify the internal os–placenta relationship when transabdominal scanning is nondiagnostic. The main contraindication to these techniques is ruptured or bulging membranes.35 Transvaginal ultrasound is considered safe in the setting of second and third trimester bleeding because optimal images are usually obtained with the transducer inserted only about 2.5 cm beyond the introitus and no closer than 3 cm to the cervix.93,94 Also, the angle between the transducer and the cervix is usually sufficient to prevent the transducer from inadvertently entering the cervix. Perform the technique using a 5.0–8.0 MHz endovaginal transducer covered by scanning medium and a sterile transducer cover. Begin the examination with sagittal scanning; the transducer may subsequently be rotated and its angle changed to obtain a longitudinal view of the placenta. Visualize the walls of the lower uterine segment in two orthogonal planes. Measure the os–placental distance if the inferior edge of the placenta is near the internal os. To do so, the endocervical canal, which appears sonographically as a faint, hyperechoic or hypoechoic line, must first be located. The junction between the amniotic fluid and the endocervical canal is designated as the internal os. Angle and rotate the transducer until the imaging plane contains both the internal os and the lowest part of the placenta, at which point freeze the image and make the measurement (Figure 15-20).

Translabial Approach

Perform a translabial (or transperineal) ultrasound examination by using a 3.5–5.0 MHz transducer to which scanning medium is applied, then a sterile cover, and finally a thin layer of sterile surgical lubricant over the cover. If available, a phased array transducer with a small footprint is preferable. The bladder should be empty. Place the transducer over or between the labia majora, posterior to the urethra, and anterior to the vaginal introitus. Perform scanning in a sagittal plane, with the bladder to the left on the monitor (Figure 15-21). Once the cervical os and placenta are visualized, laterally angle the transducer to image the entire surface of the cervix and lateral walls of the lower uterine segment.

Measure the os–placenta distance when a low-lying placenta is visualized. It has been proposed that placenta previa can be excluded by translabial scanning when a fetal part is visualized directly adjacent to the cervix or when the cervix is separated from a fetal part only by amniotic fluid without intervening placental tissue.37

The advantage of translabial scanning over transvaginal scanning is that it does not require an endovaginal transducer. Translabial scanning can be performed immediately after an apparently positive or nondiagnostic transabdominal scan without changing transducers, which may be an advantage in the emergency setting. However, there is no evidence that the translabial approach is safer than transvaginal ultrasound for patients with placenta previa, and transvaginal scanning is more familiar, provides better resolution, and is technically easier to perform.

Cervical Length Assessment

Cervical length assessment using ultrasound is a valuable tool in predicting preterm birth.51 The three scanning approaches available for measuring cervical length—transabdominal, transvaginal, and translabial—are identical to those described for assessing placenta previa. The transabdominal approach tends to suffer from various impediments to visualizing the entire cervix, such as large maternal habitus, intervening presenting part, or a shadow from the pubic bone. A distended bladder provides a good acoustic window, but may create an artificially lengthened cervix by pushing the anterior wall of the lower uterine segment against the posterior wall. The cervical canal is visualized posterior to the angle of the bladder (Figure 15-22). If transabdominal scanning fails to provide an adequate view of the cervix, transvaginal or translabial scanning is required.

The translabial approach to measuring cervical length generally requires more experience because proper image acquisition and interpretation are somewhat difficult (Figure 15-23). With the presenting part on the left side of the image, adjust the imaging plane so that the vagina courses directly away from the transducer between the bladder and the rectum. With the translabial approach, rectal gas can obscure the distal cervix and the pubic symphysis can obscure the proximal cervix. A left-side-down decubitus position and partially full bladder may facilitate translabial scanning. Translabial scanning of the cervix is particularly difficult prior to 20 weeks or with a posteriorly directed cervix.51

Regardless of the approach employed, cervical measurements are made in a sagittal plane, from internal os to external os. Begin by locating the endocervical canal, which may appear hypo- or hyperechoic. The internal os is located at the junction between the amniotic fluid and the endocervical canal. To locate the external os, visualize both the endocervical canal and the vaginal stripe. The cervix may be dynamic, especially in patients at risk for preterm labor, so observe it for 3–5 minutes before any measurements are made. In general, record the shortest cervical length (Figure 15-22). The normal cervix measures between 2.9 and 5 cm in length.51

Fetal Position and Number

Perform assessment of the fetal number and position by point-of-care ultrasound carefully and systematically. Missing the diagnosis of a breech presentation or twins may result in significant morbidity or mortality. Also, correctly interpreting sonographic images of a moving, near term fetus can be difficult. Document fetal position using the maternal bladder as a reference point. Scrutinize the presenting part in multiple imaging planes to confirm a cephalic or breech presentation versus a transverse lie. Deliberately scan the fetal spine in both a long axis and transverse plane (Figure 15-24). In the case of a known breech presentation, attempt to characterize it as a frank, complete, or footling breech (Figure 15-8). To avoid missing multiple gestations, systematically interrogate the entire uterine cavity and scan the full length of the fetus in two orthogonal planes.45

Placenta Previa

Optimal imaging is usually obtained by transvaginal sonography with the endovaginal transducer no closer than 3 cm to the cervix,41 and there have been no reported cases of transvaginal ultrasound precipitating or worsening hemorrhage. The sensitivity of transvaginal ultrasound for the diagnosis of placenta previa is nearly 100% (Figures 15-25 and 15-26).36,37,93–95 In a study that is particularly relevant in the emergency setting, it was demonstrated that when the distance from the placenta edge to the internal os was >2 cm, a vaginal delivery was possible in every case, and a cesarean section for vaginal bleeding was required in 7 of 8 cases in which the distance was less than or equal to 2 cm.93

Translabial or transperineal sonography is an increasingly accepted alternative to transvaginal scanning.33,96 As with transvaginal scanning, the os–placenta relationship is almost always well visualized, so it provides better diagnostic accuracy than transabdominal scanning39,97 and a full bladder is not required (Figure 15-27). In the case of complete placenta previa, however, a transabdominal ultrasound may be diagnostic (Figure 15-28).

Placental Abruption

The most consistent sonographic finding in placental abruption is hemorrhage and hematoma, the appearance of which will depend not only on its quantity and location but also on the timing of the ultrasound in relation to the onset of hemorrhage (Figure 15-29). Acute hemorrhage appears isoechoic to slightly hyperechoic relative to the highly vascular normal placenta (Figure 15-30).98,99 Gradually over 1–2 weeks, the hematoma becomes sonolucent and more easily distinguished from the adjacent placenta. Therefore, sonograms obtained 1–2 weeks after the onset of abruption may identify sonolucent hematomas that are not apparent on earlier studies. Isoechoic hematomas may be suspected if a portion of the placenta appears unusually thick or heterogeneous in texture. Because of their variable appearance, hematomas of abruption are occasionally mistaken for uterine leiomyomas. Another obstacle to sonographic diagnosis is that acute hemorrhage may spontaneously decompress to an adjacent area or into the vagina, such that the amount of retroplacental blood remaining is inadequate for visualization by ultrasound examination.27,28 It cannot be overemphasized that ultrasound is not considered the primary diagnostic modality for placental abruption.

Uterine Rupture

Uterine rupture is most often diagnosed by ultrasound when it occurs in the setting of trauma.11,100 Sonographic findings associated with uterine rupture include free intraperitoneal fluid, which may represent amniotic fluid or blood, and lack of fetal cardiac activity. In addition, the uterus may be empty, with the fetus found in the peritoneal cavity. There are numerous case reports describing the use of sonography to diagnose uterine rupture during labor.101,102 Sonographic findings include a visible defect in the uterine wall, fetal membranes intact, and ballooning through the uterine wall, subchorionic hematoma adjacent to a scar in the lower uterine segment, and evidence of blood layering within amniotic fluid. When uterine rupture during labor is strongly suspected on clinical grounds, an ultrasound examination should not delay cesarean section.10

Placenta Previa

1. Full bladder. Although it provides an acoustic window, a full bladder may create a false impression of placenta previa by pushing the anterior wall of the lower uterine segment against the posterior wall, which makes the cervix appear longer and the internal os appear closer to the placenta (Figure 15-19).

2. Myometrial contractions. These contractions may lead to a false-positive diagnosis of placenta previa by thickening and shortening the lower uterine wall.

3. Incomplete examination. A false-negative diagnosis may result from failure to examine the entire lower uterine segment, thereby overlooking placental tissue that encroaches on the lateral aspect of the internal os.

Placental Abruption

The diagnosis of placental abruption cannot be excluded by an ultrasound examination. The specificity of ultrasound is hampered by the common occurrence of prominent endometrial vessels, subchorionic cysts, and villous lakes. These sonolucent or hyperechoic structures may mimic the appearance of abruption but are of no clinical significance. Cardiotocographic monitoring is required for all patients when placental abruption is considered.

Cervical Length Assessment

1. Full bladder. A full bladder may create an artificially lengthened cervix by pushing the anterior wall of the lower uterine segment against the posterior wall (Figure 15-19).

2. Pseudodilatation of the cervix. A contracting lower uterine segment can produce pseudodilatation of the cervix. This can be distinguished from true dilatation by the following findings: length of the cervix is >5 cm; the distal cervix is normal; there is thickened myometrium adjacent to the cervix; and the dilatation passes after the contraction ceases.50

Gestational Age Estimate

The main pitfalls for gestational age estimation are failure to carefully follow the guidelines for measurement; failure to recognize the inherent variability of estimates; and measurement of femur length in an oblique plane relative to the long axis of the bone, which may result in a falsely short measurement.

Case 1

Patient Presentation

A 23-year-old obese woman presented to the ED with complaints of severe abdominal pain after a motor vehicle crash. The patient was the restrained driver in a rear impact collision. On arrival, her vital signs were blood pressure 80/50 mm Hg, heart rate 125 bpm, and respirations 22 per minute. Her physical examination revealed significant right-sided abdominal tenderness to palpation. The patient was unsure of her last menstrual period and indicated that she “might be pregnant.”

Management Course

Upon arrival, a 1-L bolus of normal saline was initiated and blood was sent for type and crossmatch. The E-FAST exam revealed normal lung sliding bilaterally and no evidence of either pleural or pericardial fluid. However, the abdominal portion of the exam revealed a significant amount of free fluid in Morison's pouch. A rapid transabdominal pelvic ultrasound exam revealed an intrauterine pregnancy with a fetal heart rate of 186 bpm. The BPD of the fetal skull indicated a gestational age of 18 weeks. Packed red blood cells and fresh frozen plasma were ordered and the patient was expeditiously transported to the operating room for exploratory laparotomy without further diagnostic testing.

Commentary

This case typified the axiom of “save the mother in order to save the baby.” The initial maternal vital signs were indicative of shock. The patient's presentation demanded immediate investigation with the E-FAST exam. A significant amount of intraperitoneal free fluid raised the concern for solid organ injury. Exploratory laparotomy was indicated because the patient was hemodynamically unstable. Measurement of gestational age at 18 weeks indicated a previable fetus; therefore, all efforts were appropriately focused on resuscitation of the mother. The fetal heart rate was indicative of fetal distress; however, optimal therapy was treatment of maternal hypovolemic shock.

Case 2

Patient Presentation

An 18-year-old pregnant woman, G1P0 with unknown gestational age, presented to the ED with complaints of painless vaginal bleeding and having not felt the baby move over the last several hours. The bleeding started 2 hours prior to presentation. She described changing four feminine pads during the last hour. The patient had not received any prenatal care. The hospital she presented to did not have an on-call obstetrician or labor and delivery capacity.

The patient's blood pressure was 100/78 mm Hg, heart rate 114 bpm, and respirations 14 per minute. She did not appear to be in distress. Physical examination revealed a gravid uterus with a fundal height 15 cm above the umbilicus. The abdomen was nontender to palpation.

Management Course

The emergency physician recognized the initial concern for placenta previa or abruption and deferred an initial pelvic exam. A large bore IV line was established and blood work was sent for CBC and type and screen. A transabdominal ultrasound was used initially to confirm fetal viability and gestational age. Fetal movement was noted and fetal heart rate confirmed at 140 bpm. Gestational age was estimated to be 36 weeks by femur length measurement. The transabdominal ultrasound exam failed to visualize the placental location. A transvaginal ultrasound was performed, and the placenta was visualized overlying the internal cervical os, confirming the diagnosis of placenta previa (Figure 15-26).

The patient remained stable and was emergently transferred to the nearest hospital with an obstetrics staff. The patient had continued bleeding and underwent cesarean section the next morning.

Commentary

The physician in this case determined fetal viability by estimating gestational age using femur length, and by observing fetal cardiac activity and measuring the heart rate. Understanding that transvaginal ultrasound was safe and accurate in the setting of placenta previa, and that digital examination of the cervix was contraindicated, the physician was able to make the diagnosis of placenta previa. By determining fetal viability and gestational age and by making the diagnosis of placenta previa, the emergency physician was able to safely, efficiently, and accurately evaluate this patient. This allowed the patient to be appropriately transferred for specialized obstetrical care. It is critical for emergency providers to consider placenta previa as an etiology for vaginal bleeding in the second and third trimesters of pregnancy.

Case 3

Patient Presentation

A 23-year-old woman, G2P1 at 32 weeks gestation, presented to the ED after having fallen down two steps. She fell on her side and did not strike her abdomen. She complained initially of abdominal cramping that resolved prior to arrival. Her blood pressure was 114/80 mm Hg, hear rate 98 bpm, respirations 14 per minute. Her abdominal examination revealed a gravid uterus with minimal tenderness to palpation and no other signs of trauma.

Management Course

The emergency physician performed a point-of-care E-FAST exam and did not note any abnormalities. Fetal viability and movement were noted on point-of-care transabdominal pelvic ultrasound, with a fetal heart rate of 158 bpm. No placental abruption was visualized. The patient was transferred to labor and delivery for cardiotocographic monitoring. After 6 hours without signs of contractions or fetal distress, the patient was discharged with routine obstetric follow-up.

Commentary

It is important to emphasize that placental abruption can occur in the setting of minor trauma. Abdominal pain may or may not be present. Emergency care providers must be keenly aware that ultrasound is not an accurate tool for making the diagnosis of abruption. Cardiotocographic monitoring can identify signs of fetal distress or subclinical uterine contractions that are not noticed by the patient or detectible on physical examination. Six hours of cardiotocographic monitoring without abnormal findings essentially rules out placental abruption.

The authors acknowledge Drs. Bradley Frazee, Chandra Auban, Katie Bakes, and Eric Snoey for their contributions to prior editions of this chapter, and Dr. Alfred Abuhamad and Stephanie Greenside for their contributions to this edition.