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Emergency-physician-performed US is established as a safe, rapid, and nonradiating form of diagnostic imaging for managing gynecologic and obstetric complaints and has become the standard of care for patients presenting with abdominal, pelvic pain, and/or vaginal bleeding during the first trimester. Exclusion of ectopic pregnancy during the first trimester by visualization of an intrauterine pregnancy (IUP) is the most common application. In the hemodynamically unstable patient, FAST can allow for visualization of free fluid, allowing rapid identification of patients requiring immediate surgical intervention for suspected ruptured ectopic pregnancy. The differential diagnosis is also assisted by identification of other acute gynecologic pathologies (ie, ovarian torsion, tubo-ovarian abscess, or pelvic mass).
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Positioning and Technique
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Use a low-frequency (1–5 MHz) curvilinear or phased-array probe for transabdominal imaging and a high-frequency (4–7 MHz) endocavitary probe for transvaginal imaging. With the patient in the supine position, obtain transabdominal imaging in both longitudinal and transverse planes; then with the probe marker toward the head, obtain a longitudinal view of the uterus. Rotate the probe counterclockwise, so that the probe indicator is pointed toward the patient’s right side to obtain a transverse view of the uterus. If pregnancy is early and an IUP cannot be visualized well on abdominal US or if better visualization is desired, proceed to transvaginal imaging and scan completely through the uterus and adnexal regions. Obtain multiple views of the uterus beginning with the longitudinal axis (sagittal plane) looking for the endometrial stripe, and scan the entire length of the uterus from left to right, as well as laterally to visualize the adnexa. Turn the probe counterclockwise with the probe indicator directed to the patient’s right side to visualize the uterus in the transverse plane. Determine whether there is evidence of an IUP (ie, yolk sac or fetal pole) and correlate US findings with quantitative assessment of beta human chorionic gonadotropin (β-hCG) blood levels as follows:
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With a β-hCG between 1500 to 3000 mIU/mL, a yolk sac should be visualized on transvaginal US.
With a β-hCG between 6000 to 6500 mIU/mL, a yolk sac should be visualized on transabdominal US.
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If an IUP is appreciated, organized cardiac activity should be sought if a fetal pole is present. Fetal heart movement can be captured using a video clip or by using M-mode US.
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The normal prepubertal uterus has a tubular configuration on US without an apparent endometrium and a length of 3 to 4 cm with a thickness that does not exceed 10 mm. A pubertal uterus has a pear-shaped configuration with average length of 5 to 9 cm and width of 3 to 4 cm. Endometrial lining varies with the menstrual cycle. The normal prepubertal ovary measures 1 cm in length by 1 cm in height by 1 cm in depth on US. A pubertal ovary measures 3 cm in length by 2 cm in height by 2 cm in depth.
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Ovarian torsion is more common with a predisposing lesion, such as an ovarian cyst or mass. An 8- to 12-mm mean gestational sac diameter should contain a yolk sac. An 18- to 22-mm mean gestational sac diameter should contain a fetal pole. A 5-mm fetal pole should contain a fetal heartbeat.