The presence of an adnexal gestational sac with a fetal pole and cardiac activity is the most specific but least sensitive sign of ectopic pregnancy, occurring in only 10% to 17% of cases (135,152,153). The recognition of other characteristics of ectopic pregnancy improves ultrasonographic sensitivity. Adnexal rings (ﬂuid sacs with thick echogenic rings) that have a yolk sac or nonliving embryo are accepted as specific ultrasonographic signs of ectopic pregnancy (154). Adnexal rings are visualized in 22% of ectopic pregnancies using transabdominal ultrasonography and in 38% using transvaginal ultrasonography (137). Other studies identified adnexal rings in 33% to 50% of ectopic pregnancies (135,153). The adnexal ring may not always be apparent because bleeding around the sac results in the appearance of a nonspecific adnexal mass.
Complex or solid adnexal masses are frequently associated with ectopic pregnancy; however, the mass may represent a corpus luteum, endometrioma, hydrosalpinx, ovarian neoplasm (e.g., dermoid cyst), or pedunculated fibroid (4,155–157). The presence of free cul-de-sac ﬂuid is frequently associated with ectopic pregnancy and is no longer considered evidence of rupture. The presence of intra-abdominal free ﬂuid should raise concern about tubal rupture (158,159).
Accurate interpretation of ultrasonography findings requires correlation with the hCG level (discriminatory zone) (146,151,154,160). All viable intrauterine pregnancies can be visualized by transabdominal ultrasonography for serum hCG levels higher than 6,500 mIU/mL; none can be seen at 6,000 mIU/mL. The inability to detect an intrauterine gestation with serum hCG levels higher than 6,500 mIU/mL indicates the presence of an abnormal (failed intrauterine or ectopic) pregnancy. Intrauterine sacs seen at hCG levels below the discriminatory zone are abnormal and represent either failed intrauterine pregnancies or the pseudogestational sacs of ectopic pregnancy. If there is no definite sign of an intrauterine gestation (the empty uterus sign) and the hCG level is below the discriminatory zone, the differential diagnosis includes the following considerations: 1.
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The discriminatory zone is lowered progressively with improvements in ultrasonography resolution. Discriminatory zones for transvaginal ultrasonography are reported at levels from 1,000 to 2,000 mIU/mL (146,151,154,160). Discriminatory zones vary according to the expertise of the examiner and capability of the equipment.
Although the discriminatory zone for intrauterine pregnancy is well established, there is no such zone for ectopic pregnancy. Levels of hCG do not correlate with the size of ectopic pregnancy. Regardless of how high the hCG level may be, nonvisualization does not exclude ectopic pregnancy. An ectopic pregnancy may be present anywhere in the abdominal cavity, making ultrasonographic visualization difficult.