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Ultrasonographic Imaging in the Diagnosis of Ectopic Pregnancy

Practice Essentials

Clinicians should consider the diagnosis of ectopic pregnancy in any woman in the first trimester of pregnancy who has abdominal or pelvic pain, vaginal bleeding, or both.
  The incidence of ectopic pregnancy is 1-2% and is still the most common cause of pregnancy-related death in the first trimester, accounting for about 10% of all pregnancy-related deaths.
Missed ectopic pregnancy is a leading cause of emergency medicine malpractice claims.

High-risk features for the possibility of ectopic pregnancy include history of ectopic pregnancy, history of pelvic inflammatory disease, use of an intrauterine device, and history of tubal surgery. Unfortunately, clinical findings alone cannot reliably diagnose or exclude ectopic pregnancy.

Bedside ultrasonography is indicated in the presence of vaginal bleeding or abdominal pain in a patient in the first trimester of pregnancy. Do not perform bedside ultrasonography if it delays resuscitation or definitive surgical care in an unstable patient.

Several studies have validated pelvic ultrasonography in the acute care setting, specifically in the emergency department (ED), as diagnostically accurate and beneficial for flow.

One meta-analysis of emergency physician–performed ultrasonography as a diagnostic test for ectopic pregnancy found that it had a sensitivity of 99.3% and a negative predictive value of 99.96% for detecting an intrauterine pregnancy. Given a disease prevalence of 7.5% and a negative likelihood ratio of 0.08, visualization of intrauterine pregnancy by an emergency physician yields a posttest probability of ectopic pregnancy of 0.6%.
A retrospective study of 585 women over a 2.5-year period concluded that the sensitivity and specificity of ultrasound for the detection of ectopic pregnancy was 88.5% and 96.5% on the first ultrasound and 93.1% and 95.7% after an additional scan.

In a first-trimester study of ultrasound features for diagnosis of ectopic pregnancy, an empty uterus was found to predict an ectopic pregnancy with a sensitivity of 81.1% and a specificity of 79.5%. Sensitivity and specificity for a pseudosac, adnexal mass, and free fluid were as follows: 5.5% and 94.2%; 63.5% and 91.4%; and 47.2% and 92.3%, respectively.

Bedside ultrasonography is an important tool for emergency medicine clinicians and other acute care clinicians to use in assessing patients’ risk for potential ectopic pregnancy. Early diagnosis can be very valuable in lessening morbidity and mortality. Diagnosis before tubal rupture can prevent life-threatening hemorrhage and increase the probability that the patient may be managed medically or via tube-conserving surgery.

However, use of ultrasonographic imaging should never preclude adequate resuscitation or definitive surgical therapy in a patient who is hemodynamically unstable and in whom ectopic pregnancy is strongly suspected.

The goal of bedside ultrasonography is to diagnose an intrauterine pregnancy (IUP). Ectopic pregnancy can be reliably excluded in patients with a demonstrated IUP; heterotopic pregnancy remains very rare in patients who are not taking fertility agents. Heterotopic pregnancies occur in approximately 1 in 5,000 pregnancies, but the incidence increases to as high as 1 in 100 in women undergoing fertility stimulation or procedures.

This limited diagnostic focus differs from that of the ultrasonography performed by the radiology department and has also been called point-of-care limited ultrasonography (PLUS). When the serum level of beta human chorionic gonadotropin (β-hCG) is higher than 1500 mIU/mL, the level known as the discriminatory zone, transvaginal ultrasonographic findings of an IUP should be present (see the image below).

Transverse picture of intrauterine pregnancy.

Transverse picture of intrauterine pregnancy.

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Diagnostic, Suggestive, and Indeterminate Ultrasonographic Findings

The first developmental structure big enough to be visualized by transvaginal ultrasonography is the gestational sac, which appears in the endometrial cavity at around 4.5-5 weeks’ gestation (corresponding to a β-HCG level of 1000-1500 mIU/mL). Measurement of the mean sac diameter (MSD) is important for estimating the gestational age, as well as for confirming subsequent normal embryonic development.

According to a study by Oh et al, there was no difference in gestational sac diameter at 28-35 days after the last menstrual period in normal and abnormal pregnancies, but smaller-than-expected sac diameter in pregnancies 36-42 days after the last menstrual period was found to be predictive of spontaneous miscarriage.

A conservative definition of a sonogram diagnostic for an IUP involves demonstration of a clearly defined yolk sac within the gestational sac (see the images below).

Picture of gestational sac with yolk sac.

Picture of gestational sac with yolk sac.

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Transverse picture of gestational sac with yolk sa

Transverse picture of gestational sac with yolk sac.

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The yolk sac appears by 5-6 weeks’ gestation and should definitely be present when the MSD is greater than 8 mm. The embryo, or fetal pole, can be visualized on transvaginal ultrasonography by 6 weeks’ gestation and on transabdominal ultrasonography by 7 weeks’ gestation, and it should be present when the MSD exceeds 16 mm.

Studies have shown that a cutoff of 25 mm can increase sensitivity to 100%.

Embryonic cardiac activity starts to be visible at around 7 weeks’ gestation and should be visible if the crown-rump length, or fetal pole length, is greater than 5 mm.

Definitive ultrasonographic diagnosis of an ectopic pregnancy is made in only about 20% of cases, when an extrauterine pregnancy is clearly identified (ie, an extrauterine gestational sac with a yolk sac or fetal pole is visualized). There exist, however, numerous findings that are highly suggestive of ectopic pregnancy, including an empty uterus in a patient with a β-hCG level above the discriminatory zone, an adnexal mass other than a simple cyst (see the image below), echogenic fluid in the cul-de-sac, or anything more than a small amount of fluid in the cul-de-sac.

Picture of uterus without a fetal pole and a compl

Picture of uterus without a fetal pole and a complex adnexal mass consistent with ectopic pregnancy.

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Patients who exhibit such findings should be managed in consultation with an obstetrician; they likely will need surgical exploration or medical therapy with methotrexate.

Ultrasonographic findings that are neither diagnostic nor highly suggestive of an IUP or ectopic pregnancy are classified as indeterminate. These findings include an empty uterus, an abnormal gestational sac (eg, a sac with an irregular border or an MSD large enough that a secondary structure such as a yolk sac would be expected), a normal gestational sac without a yolk sac or embryo, a nonspecific intrauterine fluid collection, and ill-defined echogenic material within the endometrial cavity (see the image below). Patients who exhibit these findings are generally monitored closely with serial β-HCG testing and clinical assessments, as about 10-25% of such patients have normal pregnancies.

Picture of abnormal endometrium in a patient with

Picture of abnormal endometrium in a patient with an ectopic pregnancy.

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