Chapter 130. Ultrasound in Pregnancy

Abdominal pain and vaginal bleeding are common complaints of patients presenting to the Emergency Department (ED) in the first 20 weeks of pregnancy. These patients account for about 1% of all ED visits. One in every 13 women presenting with these symptoms to the ED during early pregnancy will eventually be diagnosed with an ectopic pregnancy.1–5 History and physical examination have proven to be unreliable in excluding the presence of an ectopic pregnancy. Pelvic ultrasound (US) is the diagnostic test of choice in the initial evaluation of these patients with possible ectopic pregnancy.

Emergency Physicians have been using bedside pelvic US in the evaluation of first-trimester pregnancy symptoms for more than a decade. Prior studies have demonstrated that ED pelvic US decreases cost, patient length of stay, and morbidity.6,7 The goal of ED US is to identify an intrauterine pregnancy (IUP). An IUP is identified 60% to 70% of the time on bedside US in women presenting with abdominal pain and/or vaginal bleeding to ED.8 The accuracy of pelvic US performed by Emergency Physicians for identifying an IUP has been well established.9,10 With increasing experience, the scanning skills of Emergency Physicians have extended beyond just identifying an IUP. Recent studies have shown that Emergency Physicians can accurately diagnose ectopic pregnancy using bedside US.11

The pelvic cavity extends from the iliac crests superiorly to the pelvic diaphragm inferiorly. The female pelvis consists of the genital tract (vagina, uterus, and uterine tubes), ovaries, the urinary bladder, a portion of the ureters, the lower intestinal tract, pelvic musculature, ligaments, and peritoneal spaces.

The uterus is an oval, hollow, muscular, and pear-shaped extraperitoneal organ located in the pelvis between the urinary bladder anteriorly and the rectum posteriorly (Figure 130-1). It is divided anatomically into the fundus, the body, the isthmus, and the cervix. The central cavity of uterus opens into a fallopian tube on either side and into the vagina below. The uterus is usually anteverted (uterus pointing forward toward the anterior abdomen) and anteflexed (flexed forward over the bladder). It may, however, be retroverted (uterus pointing back toward the spine) and retroflexed (flexed away from the bladder) in some patients. The long axis of the uterus rarely lines up exactly with the long axis of the body. Most often it is found angled to one side. The size of the uterus is variable depending on the patient's parity, pubertal stage, and age. A postpubertal adult female uterus is approximately 8 cm long, 5 cm wide, and 3 cm deep. The endometrial canal is formed by the central, linear, and opposing surfaces of the endometrial cavity. The endometrial canal continues as the endocervical canal inferiorly. The vagina extends from the cervix to the external genitalia and lies between the urinary bladder and the rectum (Figure 130-1A).

The fallopian tubes are bilateral coiled muscular tubes located lateral to the uterus (Figure 130-1B). They are anteromedial to the ovaries and posterior to the urinary bladder. Each fallopian tube is approximately 10 cm in length and 3 mm in diameter. It is divided into four anatomical segments from proximal to distal: interstitial, isthmus, ampulla, and infundibulum.

The ovaries are bilateral almond-shaped structures that measure approximately 4 cm in length, 3 cm in width, and 2 cm in height. The size of the ovaries varies depending on the patient's age, menstrual status, and menstrual cycle phase. They are quite mobile and their position is variable. They are most often identified posterolateral to the uterus against the pelvic side walls, just anterior and medial to the internal iliac vessels within the adnexa (Figure 130-1B).

The female pelvis has an anterior cul-de-sac and a posterior cul-de-sac (Figure 130-1A). The anterior cul-de-sac or vesicouterine pouch is the peritoneal space between the anterior wall of the uterus and urinary bladder. The posterior cul-de-sac or pouch of Douglas is the area between the uterus and the rectum. The posterior cul-de-sac is the most dependent area of the peritoneal sac. A small amount of free fluid in the posterior cul-de-sac is a normal physiologic finding. A large amount of fluid in the posterior cul-de-sac is considered abnormal. Any amount of fluid identified in the anterior cul-de-sac is also considered abnormal.

The indications to perform a pelvic US examination during early pregnancy include: abdominal pain, pelvic pain, vaginal bleeding, dizziness, syncope, trauma, hypotension, or a pelvic mass on physical examination.

There are no absolute contraindications for a pelvic US. A recent pelvic surgical procedure is a relative contraindication. Always perform a bimanual or speculum examination before performing an endovaginal US examination. Use caution when performing an endovaginal US examination if the cervical os is open. The US probe should never enter the endocervical canal or the uterus.

• Ultrasound machine
• Curvilinear (2 to 5 MHz) transducer (Figure 130-2A)
• High frequency (5 to 9 MHz) endocavity transducer (Figure 130-2B)
• Condom or US probe sheath (latex and nonlatex)
• US gel

Obtain a relevant history from the patient prior to performing an US examination. It should include the following: current symptoms, date of last menstrual period, use of fertility agents, assisted reproductive technology, gravidity, parity, complications of prior pregnancies, and prior pelvic surgeries. If the patient is allergic to latex, use a nonlatex condom or probe sheath for the US examination. Discuss with the patient what the US examination entails and obtain verbal consent. A male sonographer should always be accompanied by a female chaperone when performing a transvaginal US examination.

A full urinary bladder is essential for a transabdominal US examination. The full bladder displaces bowel loops from the pelvis and provides an acoustic window to visualize the underlying pelvic organs. An excessively distended bladder can compress or displace the pelvic structures out of view. If this happens, instruct the patient to partially empty their bladder. Transvaginal US requires an empty urinary bladder. A distended bladder displaces the uterus and ovaries, creates artifacts, and makes it difficult to visualize the pelvic structures.

Transabdominal Sonography

Two approaches are used to perform a pelvic US examination of an early pregnancy: transabdominal and transvaginal. In a majority of cases, transvaginal sonography (TVS) is used in conjunction with transabdominal sonography (TAS) since both techniques provide complimentary information. TAS usually precedes TVS while the patient has a full bladder. TAS helps define the spatial orientation of the pelvis. The field of view is larger with TAS compared to TVS, allowing better visualization of the relationship of the uterus with adjacent pelvic and abdominal structures. Hemoperitoneum and large pelvic masses which extend outside of the true pelvis are better visualized with TAS. TVS is not required if the TAS provides adequate information to make a diagnosis.

TAS of the Uterus and Vagina in the Sagittal Plane

A low frequency, 2 to 5 MHz, curvilinear probe is typically used for TAS. It can also be performed using a 1 to 5 MHz phased array probe if a curvilinear probe is not available. Visualize the uterus and adnexal structures in both the sagittal (longitudinal) and coronal (transverse) planes. Place the patient supine. Apply US gel to the suprapubic area and to the foot print of the probe. Begin scanning in a sagittal or longitudinal plane by placing the probe just above the pubic symphysis. Aim the probe indicator toward the patient's head (Figure 130-3). The indicator on the probe corresponds with the left side of the screen and cephalad structures are seen on the left side of the image in the sagittal plane (Figure 130-4).

Identify the uterus and vagina in the sagittal or long axis (Figure 130-4). Note the position of the uterus and whether it is anteverted or retroverted. The shape and position of the uterus are extremely variable. The position changes depending on the amount of urine in the urinary bladder. Obesity and a retroverted position can make it difficult to identify the uterus. Identify each of the anatomic areas of the uterus: the fundus, the body, and the endometrial stripe.

The normal uterus has a sonographically low-gray homogenous texture. The appearance of the uterine endometrium changes with the different phases of the menstrual cycle. The normal endometrial stripe or endometrial canal is visible as a thin and bright echogenic line in the middle of the uterus (Figure 130-4). The sonographic appearance of the endometrial stripe changes as the thickness of the endometrium changes cyclically with the menstrual cycle. Use the direction of the endometrial stripe to find the uterine position and determine if it is anteverted or retroverted. The cervix has a similar appearance to that of the uterus.

Adjust the probe to visualize the uterine body and fundus in the same scanning plane with the vagina and cervix. If the vagina and cervix are not well visualized, slide the probe to the right and/or left of midline, angle the probe toward the patient's feet, or combine both maneuvers until the vagina and cervix come into view. The vagina is visualized between the anechoic bladder anteriorly and the echogenic rectum posteriorly (Figure 130-4). The muscular vaginal walls appear low gray and isoechoic to the uterine myometrium. The central mucosal lining of the collapsed vaginal canal walls appears thin, linear, and hyperechoic. The vagina is visible as a tubular extension of the uterus in the sagittal plane. It is flattened and oval shaped in the coronal plane. The rectum usually has air and fecal material casting a posterior acoustic shadow in the far field of the image. The bladder is always visualized in the right upper corner of the image in transabdominal view. A fully distended bladder is visible as an anechoic structure with hyperechoic walls anterior to the uterus in the near field of the image (Figure 130-4A). The bladder cavity will not be visible if it is empty and collapsed.

Twist the probe varying degrees to align the long axes of the uterus, vagina, endometrial canal, endocervical canal, and vaginal canal. Move the probe from side-to-side through the entire width of the uterus in the sagittal plane looking for evidence of an IUP. After the long axis of the uterus and vagina are determined, sweep the probe from left-to-right and right-to-left, scanning through and beyond the lateral boundaries of the uterus, the adnexa, and the pelvic side walls on both sides. Note the location of the ovaries if they are visualized. The uterus serves as the reference point for other pelvic structures. It might be necessary to apply firm pressure with the US probe to the patient's abdominal wall to displace bowel gas and improve image quality.

TAS of the Uterus and Vagina in the Coronal Plane

To scan in the coronal or transverse plane, rotate the US probe 90° counterclockwise (Figure 130-5). The probe indicator will be aimed toward the patient's right side. The probe indicator and the marker on the screen are aimed in the same direction. Angle the probe inferiorly to identify the coronal section of the vagina between the bladder anteriorly and the rectum posteriorly. Adjust the probe as necessary to identify the vaginal canal. Angle the probe inferiorly and scan through and beyond the margins of the vagina and pelvic cavity. Slowly straighten the probe back to the perpendicular position. Continue scanning superiorly through and beyond the margins of the vagina and onto the cervix. The coronal section of the cervix will appear slightly larger than the vagina. Adjust the probe to visualize the endocervical canal.

Keep the probe perpendicular and angle it further superiorly to scan through and beyond the cervix and onto the uterine body (Figure 130-6). The body of the uterus will appear larger than the cervix. Twist the probe gently to visualize the centrally located endocervical canal. Continue to scan superiorly through the body of the uterus and onto the fundus. The coronal section of the uterine fundus will appear slightly larger than the body. Adjust the probe as necessary to visualize the centrally located endometrial canal. Angle the US probe further superiorly through the fundus and urinary bladder walls up to the level of the umbilicus.

TAS of the Ovaries and Adnexa

Normal ovaries are often not visualized with TAS. They are relatively small and hidden by bowel gas or other pelvic structures with similar echogenicity. The location and lie of the ovaries are also quite variable. The long axis of the ovary can lie within either scanning plane. The ovaries are usually found lateral to the body or fundus of the uterus. Sweep laterally from the uterus in both planes to find the ovaries. Look for the greatest dimension of the ovary regardless of the probe position. The greatest dimension may be in the coronal, sagittal, or oblique plane. Determine the volume of the ovary by measuring the length, width, and height using views obtained in two orthogonal planes. Blood vessels can be confused with ovarian follicles. Rotate the probe 90° and determine if the structure becomes “tube like.” If so, it is most likely a blood vessel. Doppler can help to make this distinction.

It is difficult to thoroughly evaluate the adnexa on TAS. The ovaries can serve as landmarks for assessing adnexal pathology. Survey the adnexa for abnormalities such as masses and dilated tubular structures. If an abnormality is found in the adnexa, note its size, sonographic appearance, and its relationship to the uterus and ovaries.

Evaluate the cul-de-sac for free fluid, a hematoma, or a mass in both the sagittal and coronal planes. Fluid in the cul-de-sac is visible as an anechoic area. Note the echogenicity of the fluid. If a mass is visualized, note its size, shape, location, sonographic appearance, and relationship to the ovaries and uterus. Bowel can look like a pelvic mass, an ovary, or a dilated fallopian tube. To distinguish a bowel loop from other structures, hold the US probe still over the structure and observe it for signs of peristalsis. Differentiation of normal bowel loops from a mass may be difficult by TAS. TVS might help to differentiate a suspected mass from fluid and fecal material within the normal rectosigmoid colon.

Transvaginal Sonography

TVS is necessary after TAS when pelvic structures require further evaluation, structures are not visible on TAS, or the TAS is inconclusive. Instruct the patient to void after completing the TAS and then perform the TVS. The endocavity transducer is placed closer to pelvic structures when compared to TAS. TVS provides more anatomic detail of pelvic organs than TAS, especially adnexal structures. It provides images of pelvic structures with higher resolution allowing the early identification of intrauterine contents and adnexal abnormalities.

Place the patient in the lithotomy position. This position allows for a full range of movement of the endocavity US probe. TVS is best performed with an empty bladder following the speculum and bimanual examination to minimize patient discomfort. Remove any tampons in the vagina during the speculum examination. Also note any vaginal or cervical lesions, vaginal wounds, bulging membranes through the cervix, or if the cervical os is open. Explain the procedure and verbally consent the patient regarding the examination.

A high frequency (5 to 9 MHz) endocavity probe is used for TVS. Apply US gel to the footprint of the probe and cover it with a condom or probe sheath (Figure 130-7). Smooth all the air bubbles away from the footprint of the probe by running a finger over the condom or probe sheath covering the distal end of the probe. This ensures smooth transmission of the US beam and prevents imaging artifacts. Apply additional US gel to the outside of the condom or probe sheath before inserting the probe into the patient's vagina.

TVS requires viewing the uterus and other pelvic structures in both the sagittal and coronal planes. Each structure must be scanned left-to-right in the sagittal plane, and up-and-down in the coronal plane. Image orientation in TVS can be challenging due to the narrow field of view, the inferior scanning approach, and variations in the position of the pelvic organs. It is important to determine the proper position of the US probe before inserting it into the vagina. Aim the probe indicator toward the ceiling to scan in the sagittal plane. Rotate the probe 90° counterclockwise with the probe indicator aimed toward the patient's right side to scan in the coronal plane.

TVS of the Uterus and Vagina in the Sagittal Plane

Gently insert the US probe downward and backward into the lower third of the vagina with the indicator directed toward the ceiling (Figure 130-8). This will position the US probe adjacent to the cervix (Figure 130-8B). Start scanning by slowly lowering the handle toward the floor to view fundus of the uterus in the sagittal plane. The fundus of an anteverted uterus will be on the left side of the screen pointing toward the anterior abdominal wall (Figure 130-9). A retroverted uterine fundus will be on the right side of the screen pointing in the direction of the posterior abdominal wall (Figure 130-10). To completely visualize the retroverted uterus, lift the US probe handle toward the ceiling, moving the US probe tip posteriorly and inferiorly.

Move the probe side-to-side and vice versa to evaluate the centrally located endometrial canal and lateral margins. Any urine in the bladder will be visible in the left upper corner of the screen (Figure 130-11). Slowly lift the US probe handle toward the ceiling to visualize the body of the uterus and the cervix. Continue moving the US probe side-to-side to evaluate the lateral margins, the centrally located endometrial canal, and the endocervical canal. It may be necessary to slightly rotate or twist the US probe to fully visualize the long axis of the uterus and the entire endometrial stripe (Figure 130-11). Thoroughly screen for any intrauterine contents. Note the contents, if any, of the posterior cul-de-sac at the level of uterine fundus, uterine body, and the cervix. The goal is to scan the entire uterus from right-to-left through its entire length.

TVS of the Adnexa in the Sagittal Plane

Continue scanning sagittally to the adnexal areas after scanning the uterus in the sagittal plane. Lower the US probe handle and identify the fundus of the uterus. Move the US probe handle toward the patient's left thigh to scan through the right adnexa. This movement angles the US beam toward the right adnexal region. Return the US probe to the midline and reidentify the fundus. Move the US probe handle toward the patient's right thigh to scan through the left adnexa. This movement angles the US beam toward the left adnexal region. Repeat these lateral sweeps through the adnexal areas at the levels of the uterine body and cervix. The uterus serves as the reference point for other pelvic structures while performing the pelvic US examination.

TVS of the Uterus and Vagina in the Coronal Plane

Scan in the coronal plane after completing TVS in the sagittal plane. Turn the US probe 90° counterclockwise from the sagittal plane (Figure 130-12). The US probe indicator will be directed toward the patient's right side. The orientation in this plane is similar to a CT scan image. The left side of the US image corresponds to patient's right side (Figure 130-13). Begin scanning by slowly lowering the handle of the US probe toward the floor. Identify the uterine fundus and the endometrial stripe. Continue to scan until completely through the uterus. Slowly lift the US probe handle toward the ceiling to scan the entire length of the uterus, the cervix, and the posterior cul-de-sac.

TVS of the Ovaries in the Coronal Plane

The ovaries are best visualized in the coronal plane while scanning the adnexal regions transvaginally. Keep the uterus to one side of the image and scan up-and-down on each side of the uterus to locate the ovaries. The ovaries appear hypoechoic to the uterine myometrium with a mid-to-low gray echogenicity. The periphery of the normal ovary represents the tunica and is hypoechoic. The stroma is visualized as a low-gray echogenic center. Ovarian follicles in the periphery are small, round, and anechoic structures which vary in size and number. The sonographic appearance, size, and location of the ovaries vary greatly depending on the patient's age, menstrual phase, and pregnancy status. Identify iliac blood vessels. The ovaries are usually located lateral to the uterus and anteromedial to the iliac vessels (Figure 130-14). When the ovary is identified, scan through the margins by moving the US probe handle up and down as necessary.

Rotate the US probe 90° and sweep the beam anterior to posterior to visualize the ovaries and obtain the longest dimension of the ovary. The iliac vessels can serve as guides to obtain the longest length of the ovary. Measure the size of the ovary by obtaining the three longest dimensions of the ovary (width, length, and height) in two orthogonal planes. Note any abnormalities of the ovary. The ovaries may not be identifiable in the presence of a large leiomyoma, in prepubertal females, and in postmenopausal females. The fallopian tubes are not routinely visible on US unless filled with fluid, filled with pus, or outlined by free intraperitoneal fluid.

TVS of the Adnexa in the Coronal Plane

After scanning the uterus and ovaries, continue the coronal scanning through the left and right adnexal regions. Identify the uterine fundus. Sweep the US probe handle toward the patient's left thigh to visualize the right adnexa. Slowly move the US probe handle from the floor toward the ceiling in order to sweep through the entire right adnexal area. Return the US probe to the midline and reidentify the uterine fundus. Sweep the probe handle toward the patient's right thigh to visualize the left adnexa. Survey the adnexal regions for any masses or dilated tubular structures. If an abnormality is found in the adnexa, note its size, sonographic appearance, and relationship to the ovaries and uterus. Doppler mode may be useful to evaluate any adnexal abnormalities, to help determine if the structure is vascular, or if there is a heartbeat.

Survey the anterior and posterior cul-de-sacs thoroughly for free fluid, hematomas, or masses in the sagittal and longitudinal planes. Note the echogenicity of the fluid. If a mass is visualized, scan it in two orthogonal planes. Note its size, shape, location, sonographic appearance, and relationship to the ovaries and uterus. TVS is very helpful to differentiate a suspected mass from fluid and fecal material within the normal rectosigmoid colon. Distinguish bowel loops from other structures. Posterior cul-de-sac fluid can be quantified into small, medium, and large in volume.12 Free fluid seen less than 1/3 of the way up the posterior wall of the uterus is classified as small (Figure 130-15). Free fluid seen 2/3 of the way up the posterior wall of the uterus is classified as moderate (Figure 130-16). Free fluid seen over 2/3 of the way up the posterior wall of the uterus is classified as large (Figure 130-17).

First Trimester US

When evaluating a patient with pain or bleeding in the first trimester of pregnancy, first determine if the patient has an IUP. If an IUP is visualized, determine the number of fetuses, their viability, and the gestational age. If an IUP is not visualized, scan thoroughly looking for any signs of an ectopic pregnancy. Approximately 70% of first trimester patients seen in the ED will have an IUP visualized with US.13 The introduction of fertility treatments has increased the incidence of heterotopic pregnancy (simultaneous intrauterine and ectopic pregnancy) and multiple intrauterine gestations. TVS is generally not performed if an IUP is found on TAS and the patient is not undergoing any fertility treatment. TVS should be performed if an IUP cannot be confirmed using TAS.

Normal IUP

Early gestational structures are generally visualized 7 to 10 days earlier using TVS compared to TAS. While performing a first trimester US scan, systematically look for the sonographic findings of an IUP as discussed in the following sections.

The Gestational Sac

A gestational sac is an anechoic fluid collection within the uterus surrounded by a ring of bright, thick, and symmetric echogenic tissue (Figure 130-18). It is clearly visible at 5 weeks of gestation by TVS. The gestational sac is approximately 5 mm in diameter at 5 weeks and grows at a rate of 1 mm/day. A gestational sac is not a definite sign of an IUP. Determine the number and location of the gestational sac(s). Evaluate the gestational sac(s) in two orthogonal planes. Note the outline, shape, and contents of the gestational sac(s). The normal gestational sac is a round, centrally located, smooth walled structure (Figure 130-18). Suspect a hematoma formation if the structure is irregular in outline and collapsing. The echogenic tissue surrounding the gestational sac represents developing chorionic villi and the adjacent endometrium. It should be thicker and echogenically brighter than the myometrium.

The size of the gestational sac can be used for gestational dating. The most commonly used method is the mean sac diameter (MSD) method. Obtain the longest three measurements (in mm) of the length, width, and height of the gestational sac in two orthogonal planes. Do not include the echogenic rim of trophoblastic and decidual tissue surrounding the gestational sac. Obtain the measurements from the inner margin to the inner margin of the gestational sac. The MSD is calculated by the following equation: MSD = (length + width + height) ÷ 3. The gestational age is calculated by the following equation: gestational age (days) = MSD (mm) + 30.

The Double Decidual Sign

Two concentric echogenic rings surrounding a gestational sac form the double decidual sign. It appears as a hyperechoic and a hypoechoic layer around the gestational sac (Figure 130-19). The outer bright ring is called the decidua vera and is formed by the endometrial lining of the uterus (the decidua). The inner echogenic ring is called decidua capsularis and is formed by the chorion surrounding the gestational sac. The two bright layers are separated by a hypoechoic or anechoic layer of fluid which is the endometrial canal. The double decidual sign is visible once the gestational sac reaches 10 mm in internal diameter. The double decidual sign is highly suggestive of an IUP, but it is not 100% reliable. It occurs in only about half of all intrauterine pregnancies.

The Yolk Sac

The yolk sac is the first embryonic structure seen within the gestational sac (Figures 130-18 & 130-19). It is the earliest reliable US sign of an IUP. The presence of a yolk sac within the gestational sac in the uterus is diagnostic for an IUP. The normal yolk sac is round with an anechoic fluid-filled center and measures less than 6 mm in diameter. It progressively increases in size to a maximum diameter of 6 mm. The yolk sac is visualized as a bright, spherical, thin-walled structure within the gestational sac with a balloon on a string appearance (Figures 130-18 & 130-19). It is visible when the gestational sac is >10 mm by TVS and >20 mm by TAS. The yolk sac is usually visible 5 to 6 weeks after the last menstrual period by TVS and at 7 weeks by TAS. Measure the yolk sac from its inner margin to its opposite inner margin. It generally disappears after 10 to 12 weeks, but can persist until 20 weeks.

The Embryo

The embryonic or fetal pole is visible at 5 to 6 gestational weeks by TVS. At this point of gestation it is approximately 1 to 2 mm in length. The normal embryo grows at a rate of 1 mm/day. It appears as a highly echogenic focal thickening adjacent to the yolk sac, hugging the wall of the gestational sac surrounded by anechoic fluid (Figure 130-18A). The embryo is visible within the gestational sac when the MSD is ≥16 mm by TVS. The presence of a fetal pole within the gestational sac in the uterus is diagnostic for an IUP. Measure the fetal pole along its longest axis.

Fetal Cardiac Activity

Fetal cardiac activity is visible by 6.5 weeks of gestation and a fetal pole size of 6 mm by TVS. Fetal cardiac activity is visible by TAS in embryos >10 mm long. A faint fluttering motion within the fetal pole adjacent to the yolk sac represents the neurologically active heart tissue. The presence of fetal cardiac activity inside the uterus is diagnostic for a live IUP. The incidence of a miscarriage, or spontaneous abortion, before 20 weeks gestational age with a live IUP diagnosed on US in the ED is 9.2%.14 Use M-mode to determine the fetal heart rate (FHR). It is calculated by measuring the length of one cycle and then determining the cycles per second from that measurement (Figure 130-20). The normal FHR for an embryo >6 mm in length is over 120 beats/min. FHR serves as an important prognostic indicator. Rates less than 90 beats/min are associated with a high risk of spontaneous abortion.

Crown Rump Length

The crown rump length (CRL) is the most accurate sonographic measurement for gestational dating between 6 and 12 weeks. It is accurate within ±4.7 days for gestational dating. At 6 weeks of gestational age it is not possible to differentiate the crown from the rump. The embryonic pole length is used for the CRL measurement. By 8 weeks a true CRL measurement can be determined. Obtain the longest measurement of an unflexed fetus and do not include the yolk sac (Figure 130-21). The gestational age is calculated by the following equation: gestational age (weeks) = CRL (mm) + 6.5.

Corpus Luteum Cyst

The corpus luteum is a normal physiologic structure found within one of the ovaries during pregnancy. It is unilocular and thin-walled. The sonographic appearance of a corpus luteum is highly variable. It can appear as a hypoechoic, hyperechoic, or isoechoic structure. Power Doppler mode shows the corpus luteum has increased blood flow at the periphery that is described as a “ring of fire” (Figure 130-22). The ring of fire is also visible with an ectopic pregnancy.

Embryonic Demise or Early Pregnancy Failure

Definitive diagnosis of an early pregnancy failure requires serial US examinations. A blighted ovum is a failed pregnancy in which the embryo failed to develop within a gestational sac of sufficient size that an embryo should be visible. There are several sonographic findings that can suggest early pregnancy failure. The earliest sign of early pregnancy failure is a gestational sac without a yolk sac or embryo (Figure 130-23A). An empty gestational sac with a MSD ≥20 mm and no evidence of an embryo or yolk sac by TVS is highly suggestive of an embryonic demise. Other predictors of embryonic demise are: a gestational sac >10 mm in diameter by TVS without a yolk sac, a gestational sac >20 mm in diameter by TAS without a yolk sac, a gestational sac >18 mm in diameter by TVS without a fetal pole, or a gestational sac >25 mm in diameter by TAS without a fetal pole.

Other signs suggestive of early pregnancy failure include a gestational sac that is irregular in shape, distorted, or collapsed (Figure 130-23B). A gestational sac positioned low in the uterine cavity or surrounded by a thin (<2 mm wide) decidual reaction indicates embryonic demise. Yolk sac abnormalities suggest an abnormal pregnancy. Absence of a yolk sac when an embryo is visualized prior to 12 weeks is indicative of embryonic demise. A large yolk sac (>6 mm in diameter) is predictive of embryonic demise. A calcified yolk sac is associated with an adverse pregnancy outcome. The absence of fetal cardiac activity in an embryo with a crown rump length >6 mm is an indicator of embryonic demise (Figure 130-24). Embryonic bradycardia is associated with a poor prognosis. Fetal heart rates <90 beats/min with a CRL >5 mm are associated with an 80% rate of eventual fetal demise.

Spontaneous Abortion

A spontaneous abortion is the most common complication of first trimester pregnancy and occurs in approximately 20% of clinically apparent pregnancies.15 An empty uterus with a clear midline echo occurs on US after a complete spontaneous abortion. An incomplete abortion is visible on US as irregular echogenic material within the uterine cavity (Figure 130-25A) or a thickened midline stripe (≥10 mm wide) within the uterine cavity (Figure 130-25B). These both represent retained products of conception.

Subchorionic Hemorrhage

A subchorionic hemorrhage is bleeding between the chorion and the uterine endometrium. It can cause sudden pelvic pain or vaginal bleeding. The overall incidence in pregnancy is 1.3%. It results in the chorionic membrane separating and elevating from the decidua vera (endometrium) by a hematoma or clot. The acute hemorrhage appears hyperechoic or isoechoic. The hematoma becomes hypoechoic or anechoic as it undergoes liquefaction during the subsequent 1 to 2 weeks (Figure 130-26).

There is an increased risk of miscarriage, stillbirth, and preterm labor in patients with a subchorionic hemorrhage. In a retrospective study done by Bennett et al., the overall pregnancy loss rate was 9.3%.16 This was found to increase with increasing maternal age and decreasing gestational age. The prognosis depends upon the size of the subchorionic hemorrhage. Small and medium sized hemorrhages, up to approximately 50% of the gestational sac circumference, have a spontaneous abortion rate of 9%. Hemorrhages larger than 50% of the gestational sac circumference have a spontaneous abortion rate of 18.8%. Hemorrhages >40% of the volume of the gestational sac are associated with spontaneous abortion rates of 50%.

Ectopic Pregnancy

It is estimated that approximately 2% of all pregnancies are ectopic pregnancies in the United States.17 The prevalence of ectopic pregnancy in the ED is much higher compared with the general population, accounting for approximately 8% of all pregnant patients seen in the ED.18,19 Approximately 40% of ectopic pregnancies are missed or not diagnosed during the initial ED evaluation. Ectopic pregnancy is still the leading cause of pregnancy-related deaths during first trimester of pregnancy.20 Clinical criteria alone are not sufficient to distinguish between a patient having a spontaneous abortion and an ectopic pregnancy. Pelvic US is the diagnostic test of choice in the evaluation of a patient with a possible ectopic pregnancy. The risk of heterotopic pregnancy in the general population is 1 in every 30,000 pregnancies. The risk increases in women undergoing fertility treatments to 1 in every 5000 pregnancies or greater.

The discriminatory zone is the β-hCG level at which an IUP should be seen by pelvic US. This level generally is between 1000 and 2000 mIU/mL by TVS and between 4000 and 6500 mIU/mL by TAS. Approximately 38% of patients with a confirmed ectopic pregnancy had a β-hCG level <1000 mIU/mL. Approximately 39% of ectopic pregnancies with β-hCG levels lower than 1000 mIU/mL can be identified on US.21 The concept of a discriminatory zone becomes more significant if the US is indeterminate.

There is no β-hCG level that rules out an ectopic pregnancy in a pregnant patient. It is important to correlate the serum β-hCG values with the US findings. The suspicion for an ectopic pregnancy should be high if the US findings are indeterminate and the serum β-hCG levels are above the discriminatory zone.

Most ectopic pregnancies (95%) occur in the fallopian tube: 80% in the ampulla, 10% to 15% in the isthmus, and <2% in the interstitial region. The remaining 5% are found in the cervix, on the ovary, and in the abdomen. Approximately 28% of pregnant patients presenting to the ED have no definite signs of either an IUP or an ectopic pregnancy on US. In these cases, the diagnosis of an ectopic pregnancy is often reached through a combination of abnormal US findings. The spectrum of sonographic findings of an ectopic pregnancy is broad and includes: an empty uterus, a pseudogestational sac, an adnexal mass, an extrauterine empty gestational sac, an extrauterine gestational sac with a yolk sac or an embryo (with or without cardiac activity), a tubal ring, and fluid in the cul-de-sac.

Empty Uterus

An ectopic pregnancy should be suspected in any pregnant patient presenting with abdominal pain or vaginal bleeding. Perform TVS to determine if an IUP is present. The empty uterus is the lack of an IUP in a pregnant patient. An empty uterus is a sign of an ectopic pregnancy unless proven otherwise.

Pseudogestational Sac

A pseudogestational sac is the decidualization of the endometrium from hormonal stimulation by the ectopic pregnancy. It occurs in 10% to 20% of ectopic pregnancies. The pseudogestational sac is often <10 mm in diameter and consists of endometrial fluid. The pseudogestational sac is distinguished from a normal gestational sac by its central location in the endometrial cavity, lack of a double decidual sign, an ovoid shape, and poorly defined margins (Figure 130-27). A normal gestational sac is eccentrically placed within the endometrial cavity.

Extrauterine Gestational Sac

An intact gestational sac with a yolk sac outside the uterus confirms an ectopic pregnancy (Figure 130-28). The most specific finding of an ectopic pregnancy is the presence of a live extrauterine pregnancy. This occurs in 3% to 26% of ectopic pregnancies.22

Adnexal Masses

Adnexal abnormalities occur in approximately 70% of ectopic pregnancies. The sonographic appearances of these abnormalities are variable, often only suggestive and not diagnostic of an ectopic pregnancy. Abnormalities are frequently visible as spherical structures with increased blood flow on Power Doppler, similar to the ring of fire seen with a normal corpus luteum. The most common finding is a small noncystic mass with mixed echoes adjacent to the ovary (Figure 130-29). The corpus luteum is located on the same side as the ectopic pregnancy in 80% of cases (Figure 130-30). Adnexal masses next to the ovary can be difficult to distinguish from a normal corpus luteum. Apply pressure with the US probe to help separate the ectopic mass from the ovary.

Tubal Ring

The tubal ring is an extrauterine hypoechoic concentric mass with a thick-walled echogenic rim and separate from the ovary (Figure 130-31). It represents a gestational sac and the surrounding trophoblastic reaction. The presence of a tubal ring has a 95% positive predictive value and a 50% sensitivity for an ectopic pregnancy. It may be visible in up to 60% of ectopic pregnancies by TVS.

Free Fluid in the Cul-De-Sac

A hemoperitoneum is found in approximately 40% to 83% of patients with a complicated ectopic pregnancy. This has a 90% positive predictive value for an ectopic pregnancy. Anything more than a small amount of pelvic fluid is abnormal. Free fluid in the cul-de-sac and an empty uterus on TVS is an ectopic pregnancy until proven otherwise (Figure 130-32). The greater the quantity of free fluid in the pelvis, the higher the likelihood of an ectopic pregnancy. As the amount of pelvic fluid increases, it spreads from the posterior aspect of the cervix to the posterior aspect of the uterus, then to above the fundus or around the ovaries. The type of free fluid in the pelvis should be taken into account to determine the risk of an ectopic. If the free fluid is echo free, the risk is dependent on the quantity of the free fluid. Any amount of echogenic pelvic fluid represents blood and a high risk for the possibility of an ectopic pregnancy. Hyperechoic masses floating within the echogenic pelvic fluid suggests clotted blood.

Free fluid in the pelvis occurs frequently with a ruptured ectopic pregnancy. Free fluid may also be found with an unruptured ectopic pregnancy. The likelihood of a ruptured ectopic pregnancy increases as the amount of free fluid increases. Clotted blood in the cul-de-sac after a ruptured ectopic pregnancy can distort anatomic landmarks and obscure the US image. Scanning through the hepatorenal space can often lead to identification of an active hemorrhage. The presence of free fluid in the Morison's pouch of a first-trimester patient without any evidence of an IUP is highly suggestive of a ruptured ectopic pregnancy.

Perform a complete ED US examination. Classify the US findings into one of three categories: IUP, ectopic pregnancy, or indeterminant. Determine the patient disposition upon their symptoms, the US results, and the β-hCG level as discussed in the following section.

Patient disposition is based upon their vital signs, physical examination findings, and bleeding status when the definitive diagnosis of an IUP is made and patient did not undergo any fertility treatments. If the patient is hemodynamically stable with no significant abdominal tenderness and not actively bleeding, discuss the risks and symptoms of a spontaneous abortion and instruct the patient to follow-up with an Obstetrician. If the patient has severe abdominal pain, consider other causes of abdominal pain such as ovarian torsion, appendicitis, cholelithiasis, renal colic, etc. A complete spontaneous abortion can be managed expectantly with follow-up by an Obstetrician. An incomplete abortion requires an ED obstetric consultation for possible dilatation and curettage. Consult an Obstetrician if there are definite signs of an ectopic pregnancy on US. The Obstetrician will determine the need for chemical or surgical treatment.

An indeterminate US study demonstrates no signs of an IUP or an ectopic pregnancy. In a prospective study done by Tayal et al., 20% of all ED first trimester pelvic US examinations were categorized as indeterminate.23 Approximately 20% of patients with an indeterminate US study and β-hCG level >1000 mIU/mL were found to have an ectopic pregnancy.24,25 The possible outcomes with indeterminate first trimester US studies are early pregnancy, ectopic pregnancy, missed abortion, or an incomplete abortion. If a patient with an indeterminate US study is hemodynamically stable with no significant abdominal or adnexal tenderness, no free fluid on US, and a β-hCG level <1000 mIU/mL, they can be safely discharged with follow-up in 48 hours for reevaluation.

There is no specific aftercare for the patient based on the US examination itself. All disposition and management decisions are described in the assessment section above. Dispose of the condom or probe sheath after completing the transvaginal US examination. Soak the endocavity US probe in an antimicrobial solution. Follow the manufacturer's specifications and your hospital's infectious disease department recommendation for the type of the antimicrobial solution and the soak time.

There are no complications to performing a pelvic US examination except for mild discomfort during the procedure. US examination of the fetus is generally considered to be safe during pregnancy. Prior studies have reported no association of US with any adverse fetal outcomes.

Women with abdominal pain and bleeding during the first trimester of pregnancy are commonly seen in the ED. Pelvic US is the primary imaging modality used in the evaluation of first trimester symptoms. ED US has revolutionized the assessment of these patients. The goals of the first trimester US are to find an IUP and rule out an ectopic pregnancy. Knowledge of sonographic anatomy, technique, and limitations are essential to perform this procedure. Understanding the principles discussed in this chapter and following a systematic approach to performing a pelvic US examination will maximize the amount of useful information obtained in first trimester patients.