Ultrasound in Obstetric Emergencies

Carla Sterling

Zachary Testo

Gavin Budhram

OVERVIEW

Ultrasound was first used to portray a fetus in utero in 1953 and subsequently became the preferred imaging modality in obstetric emergencies due to its ready availability, excellent resolution of moving fetal parts in real time, and lack of ionizing radiation.¹ Although limited by operator skill and patient habitus, ultrasound has remained the most important advancement in obstetrics in the last century.

Up to twenty-five% of first trimester pregnancies will present to the emergency department (ED) with vaginal bleeding or pelvic pain and ultrasound is the primary modality to rule out an ectopic pregnancy.²^,³^,⁴^,⁵ Ultrasound is also helpful in the evaluation of pregnancy- and nonpregnancy-related abdominal pain, vaginal bleeding, and trauma as well as for patients presenting with signs of labor. This chapter focuses on goal-directed, clinician-performed ultrasound evaluations for common obstetric emergencies.

SCANNING TECHNIQUES

Transabdominal Pelvic Ultrasound

Transabdominal pelvic ultrasound is the primary technique for imaging the fetus and is capable of identifying the first sign of pregnancy—a gestational sac—as early as 5.5 weeks. This technique can visualize the uterus and surrounding structures and is ideally performed with a full bladder. A curvilinear transducer (Figure 8.1) is used owing to its large footprint and low frequency, providing a wider and deeper field of view. This is useful in late gestation as the fetus grows larger and cannot be visualized with transvaginal ultrasound alone. On most ultrasound machines, it is important that an “OB” or “obstetrics” preset be used so that the clinician has access to fetal calculations such as heart rate and crown-rump length.

First-Trimester Imaging

The patient should be placed in the supine position with legs extended. To best visualize the uterus, place the transducer in the sagittal plane superior to the pubic symphysis. The transducer indicator should point toward the patient’s head (Figure 8.2). The examiner then sweeps the transducer left and right until the bladder, uterus, and surrounding structures are identified
(Figure 8.3). The structures on the left of the ultrasound images are cephalad and the structures on the right are caudad. Anterior structures, most often the bladder, will appear at the top of the image; deep to the bladder lies the uterus. Intestines and free fluid may be visualized in the spaces around these structures. The uterus is a thick-walled structure just inferior to the wall of the bladder, with the fundus and body located anterior to the cervix. The endometrial stripe should be identified running down the middle of the uterus to the cervix. The cervix is visualized under the deepest part of the bladder. The vaginal canal can be identified deep to the bladder on the right of the screen.

Figure 8.1: Curvilinear transducer.

Figure 8.2: Transducer placement for sagittal pelvic window. Transducer is placed just above the pubic symphysis with the indicator cephalad and probe aimed into the pelvis. (Reproduced with permission from Nath J, ed. Female reproductive system and obstetrics. In: Programmed Learning Approach to Medical Terminology. 3rd ed. Baltimore, MD: Wolters Kluwer; 2018:520.)

An axial window is obtained by turning the transducer 90° counterclockwise so that the indicator points toward the patient’s right side. The structures seen on the left of the ultrasound image correspond with the patient’s anatomic right. The uterus is visualized deep to
the bladder (Figure 8.4). The transducer is then swept superiorly and inferiorly to obtain coronal planes of the pelvis to identify structures including the fetus, free fluid, and any adnexal masses. The ovaries are located between the uterus and iliac arteries and are best visualized in this plane. Identification of ovaries, however, can be difficult using transabdominal ultrasound due to body habitus, displacement by an enlarged uterus in advanced pregnancy, and normal anatomic variation.

Figure 8.3: Sagittal (longitudinal) pelvic window.

Second- and Third-Trimester Imaging

At 12 weeks gestation, the uterus moves out of the pelvis into the abdominal cavity. As the uterus enlarges during the second and third trimesters, identification of fetal parts is more variable and is dependent on fetal position. Imaging planes (e.g., sagittal, coronal, and axial) are usually described in relation to fetal lie. To identify structures in a longitudinal plane, rotate the transducer until the spine is identified running along the posterior aspect of the fetus. The spine is brightly echoic with posterior shadowing and is readily identifiable (Figure 8.5). The transducer can then be swept laterally and medially to identify structures. The transducer is then rotated 90° counterclockwise to obtain transverse images and swept inferiorly and superiorly. The skull is identified in a transverse plane to measure the biparietal diameter (BPD; Figure 8.6).

Figure 8.4: Axial (transverse) pelvic window.

Figure 8.5: Fetal spine in later pregnancy indicating fetal lie.

Figure 8.6: Biparietal diameter showing gestational age of 22 weeks and 5 days.

Ultrasound is a safe and accurate method of assessing the cervix for signs of labor. Although traditionally evaluated by digital examination, ultrasound may be safer if placenta previa or ruptured membranes are present. There is evidence that an ultrasound examination is more reliable than a digital examination for cervical assessment.⁶ The cervix is best imaged in long axis inferior to the bladder. Cervical dilation and length can be measured, with length calculated from internal os to external os (Figure 8.7).

Transvaginal Pelvic Ultrasound

Transvaginal ultrasound is most useful early in pregnancy, allowing identification of an intrauterine pregnancy (IUP) as early as 4.5 weeks. The endocavitary transducer (Figure 8.8) is a high-frequency probe providing high-quality resolution images but at a shallow depth of field. Structures that are more than 8 to 10 cm from the tip of the transducer cannot be seen. Given this shallow depth of field, transvaginal ultrasound should always be performed in conjunction with transabdominal examination. Transvaginal ultrasound is useful in early fetal development when an IUP is more difficult to identify.

Transvaginal ultrasound is performed in conjunction with a pelvic examination, using the same patient positioning—with the patient supine and feet in lithotomy stirrups. Alternatively, the patient can be positioned in the frog-leg position, with the pelvis elevated. Gel is applied to the transducer’s footprint followed by a transducer cover and sterile water-soluble lubricant on the
outside of the transducer cover. Ideally, the patient should have a completely voided bladder to improve the quality of the images and reduce artifact.

Figure 8.7: Transabdominal window with cervical measurement.

Figure 8.8: Endocavitary transducer.

Uterus

The transducer is inserted into the vagina, with the transducer indicator pointing toward the ceiling until it seats comfortably adjacent to the cervix (Figure 8.9). The structures seen on the right of the ultrasound image correspond anatomically to the anterior structures. The uterus should be identified by sweeping the transducer left and right and then rotating slightly until the hyperechoic endometrial stripe can be seen running through the uterus into the cervix (Figure 8.10). The midline of the uterus may not directly correspond with the midline of the patient’s body. The pouch of Douglas is visualized deep to the uterus. When this view is obtained, the transducer is swept toward the patient’s left and right to image through the uterus and identify the fetus. The cervix is visualized by withdrawing the transducer a few centimeters and tilting downward into the pelvis.

Coronal images are obtained with the transducer rotated 90° counterclockwise so that the indicator is to the patient’s right (Figure 8.11). In this view, the structures on the left of the ultrasound image correspond with the patient’s anatomic right (Figure 8.12). The uterus and midline stripe are again identified. When this view is obtained, the transducer is swept anterior and posterior to visualize the coronal planes of the pelvis.

Figure 8.9: Transducer placement for sagittal (longitudinal) endocavitary window. (Redrawn from Simon B, Snoey E, eds. Ultrasound in Emergency and Ambulatory Medicine. St. Louis, MO: Mosby-Yearbook; 1997.)

Figure 8.10: Sagittal endocavitary window.

Figure 8.11: Transducer placement for coronal (transverse) endocavitary window. (Redrawn from Simon B, Snoey E, eds. Ultrasound in Emergency and Ambulatory Medicine. St. Louis, MO: Mosby-Yearbook; 1997.)

Figure 8.12: Coronal (transverse) endocavitary window showing uterus.

Ovaries

Ovaries are identified best in the coronal plane by moving the transducer toward the left or right adnexa and then sweeping anteriorly and posteriorly. They are located lateral and posterior to the body of the uterus, although the position is variable, especially in parous women. Ovaries are oval shaped and appear hyperechoic, with small, circular hypoechoic follicles contained within, commonly described as “chocolate chip” in appearance. The transducer is then rotated to assess ovarian dimensions, and diameter measurements are obtained using the electronic calipers. If ovarian torsion is of concern, the clinician may place the ovary in the middle of the screen and perform “sonopalpation.” Gentle pressure is applied directly to the ovary with the transducer to assess for reproducibility of pain. Color and Doppler assessments of ovarian blood flow may also be obtained, but are considered outside the scope of most emergency physicians.

Cervix

If the cervix is inadequately visualized by the transabdominal ultrasound approach due to an empty bladder or significant maternal obesity, the transvaginal approach may be used (Figure 8.13). The cervix is visualized in up to 100% of patients, with the main contraindication being ruptured or bulging membranes.⁷ Optimal imaging can be obtained with the transducer inserted about 2.5 cm beyond the introitus and no closer than 3 cm from the cervix.⁸^,⁹

Figure 8.13: Endocavitary window of cervix with calipers indicating length.

NORMAL ULTRASOUND FINDINGS IN PREGNANCY

First Trimester

Fetal development in utero follows a predictable sonographic progression. The first sign of an IUP is a gestational sac, appearing as a hypoechoic structure located in the fundus along the endometrial stripe. This is followed by the double decidual sign, appearing as two hyperechoic rings surrounding the gestational sac separated by a hypoechoic layer. A double decidual sign, however, is only observed in 50% of pregnancies.¹⁰ Therefore, identification of a yolk sac is considered the most reliable finding of early pregnancy. The yolk sac is a small echogenic ring inside the gestational sac and is considered confirmatory for an IUP (Figure 8.14). On transvaginal ultrasound, the yolk sac is seen at 5 to 6 weeks, typically with a β-human chorionic gonadotropin (β-hCG) level above 1,500 to 2,000. The fetal pole is the next finding on ultrasound, seen at more than 6 weeks’ gestation, and is identified in its earliest form as a small hyperechoic tissue mass adjacent to the yolk sac. Cardiac activity can be visualized as a “flicker” within the embryo. Using M-mode and preprogrammed ultrasound calculations, the fetal heart rate can be measured (Figure 8.15). A normal fetal heart rate
is between 120 and 180 beats/min. Gestational age is estimated by obtaining a crown-rump length measurement (Figure 8.16). For an accurate measurement, it is important that the transducer be rotated so that the longest dimension of the fetal pole is measured. The yolk sac should not be included in the measurements. In later stages of the first trimester, fetal movement can be visualized and should be documented if observed.

Figure 8.14: Gestational sac with a double decidual sign with yolk sac in early pregnancy.

Figure 8.15: M-mode measurement of fetal heart rate. The repetitive pattern in the middle of the tracing indicates fetal cardiac activity.

Figure 8.16: Crown-rump measurement indicating gestational age.

Second and Third Trimesters

Fetal Heart Rate

The fetal heart rate in later stages of pregnancy is measured using either a continuous-wave ultrasound machine (which produces only sound without pictures) or the M-mode technique. M-mode measurements are easier to obtain in later stages of pregnancy because the heart is larger and better visualized.

Fetal Lie

Fetal lie and head position are simple to determine with ultrasound. To assess fetal lie, the examiner should rotate the transducer until the spine is identified running along the posterior aspect of the fetus. Visualizing the spine in a longitudinal axis will indicate the fetal lie (Figure 8.5). Fetal head position is determined by following the spine until the head is encountered. The location of the ultrasound transducer when the head is centered in the middle of the screen indicates head position. Breech presentation is diagnosed if the head position is in the upper abdomen and the spine can be followed inferiorly toward the pelvis.

Fetal Age

In the second and third trimesters, gestational age is assessed by BPD. The skull is imaged in a transverse plane at the level of the third ventricle and symmetric-appearing thalami (Figure 8.6). Measurement is from the outer edge of the near calvarial wall to the inner edge of the far calvarial wall, without inclusion of soft tissues outside the skull. During active labor, the head may be difficult to see if it is engaged in the pelvis and obscured by the overlying pubic symphysis.

Placenta

The placenta appears as a uniformly echogenic (intermediate echogenicity) structure along the uterine wall, sometimes with a deep hypoechoic band separating it from normal uterine myometrium (Figure 8.17). It can be located along any uterine wall and may be difficult to see if attached
to the posterior uterine wall and shadowed by overlying spine and fetal parts. The position of the placenta should be noted in relation to the cervix, with the cervix being located just deep to the deepest part of the bladder. Placenta previa is diagnosed in the third trimester, but marginal or low-lying placental positioning is often present in the second trimester. This occurs because as the fetus grows, elongation of the uterus, mostly in the lower uterine segment, causes the placenta to move superiorly in the vast majority of pregnancies.

Figure 8.17: The placenta is uniformly echogenic and adherent to the uterine wall.

FIRST-TRIMESTER PATHOLOGY

Vaginal bleeding and pelvic pain are common presentations to the ED during the first trimester of pregnancy.²^,³^,⁴^,⁵ Ultrasound aids in the rapid bedside evaluation of these patients, allowing identification of ectopic pregnancy as well as other etiologies such as pregnancy loss, pelvic mass, ovarian torsion, and gestational trophoblastic disease. The clinician should know the limits of their bedside ultrasound skills, and referral for formal confirmatory sonography by radiology is recommended for most findings, except normal IUP or ectopic pregnancy with hemodynamic instability.

Ectopic Pregnancy

The incidence of ectopic pregnancy has increased in recent years; however, the case fatality for ruptured ectopic pregnancy has decreased dramatically.¹¹^,¹² This is primarily due to the use of ultrasound in the early stages of pregnancy facilitating rapid diagnosis and treatment. Earlier identification of ectopic pregnancy allows more conservative treatment options such as methotrexate over surgical interventions.

The main goal in the evaluation of a first-trimester pregnancy with suspicion for ectopic pregnancy is to identify an IUP. Because the rate of spontaneous heterotopic pregnancy is 1 in 4,000 to 1 in 8,000 pregnancies,¹³^,¹⁴^,¹⁵ this essentially rules out ectopic pregnancy in patients not undergoing fertility treatment. Patients undergoing fertility treatment have an incidence of heterotopic pregnancy as high as 1 in 100.¹⁶^,¹⁷^,¹⁸ Bedside ultrasound is a very sensitive modality for ruling out ectopic pregnancy (99.3%) and can be used to safely determine disposition in first-trimester patients with pelvic pain or vaginal bleeding.¹⁹ Transvaginal ultrasound can establish a diagnosis of IUP or ectopic pregnancy in 75% of patients at the time of their initial ED presentation.²⁰ There are several ultrasound findings identifiable by point-of-care ultrasound (POCUS), which should raise clinical suspicion for ectopic pregnancy.

Absent IUP

An IUP should be seen with β-hCG levels greater than 6,500 mIU/mL using transabdominal ultrasound and with β-hCG levels more than 1,000 to 1,500 mIU/mL using transvaginal ultrasound.²¹^,²² This threshold value for β-hCG is known as the discriminatory zone. A low β-hCG level does not
rule out ectopic pregnancy as 40% of ectopic pregnancies present with β-hCG measurements less than 1,000 mIU/mL and 20% present with a β-hCG less than 500 mIU/mL.²⁰^,²³ Therefore, a low β-hCG measurement and sonographically empty uterus is considered a pregnancy of unknown location. Clinical suspicion should remain high for possible ectopic pregnancy, and strict discharge precautions and follow-up should be provided. An ultrasound should always be performed even if the β-hCG level is below the discriminatory zone, as ectopic pregnancy or secondary findings of ectopic pregnancy are commonly visualized at low β-hCG levels.

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