Less than 10% of ectopic pregnancies occur in the cervix, the cornua, the ovary, or the abdomen. These are more difficult to diagnose and treat, thus resulting in higher morbidity than tubal pregnancies. More recently, implantation in Cesarean section scars has been described with increasing frequency.

An empty-appearing uterus may indicate an intrauterine pregnancy too early to see (less than 5 weeks), a failed pregnancy too small or already passed, or an ectopic pregnancy. Technical factors such as fibroids, adenomyosis, morbid obesity, or an axial uterus may make imaging more difficult. Approximately 7% to 20% of women presenting with a PUL are ultimately diagnosed with ectopic pregnancy.

When there is no visible intrauterine pregnancy, it is important to determine the level of human chorionic gonadotropin (hCG). Serial hCG levels are necessary when evaluating pregnancies of unknown location to observe the trend. In a normal early pregnancy, hCG levels will rise by at least 53%, but typically greater than or equal to 100% (99% confidence interval) every 48 hours. Most ectopic pregnancies are associated with a low and abnormally slow rising hCG level. The reported “discriminatory hCG value” at which an intrauterine pregnancy must be identified sonographically has varied in the literature between 1000 and 3000 mIU/mm. It is probable that a patient with an empty uterus and an hCG level of greater than 3000 mIU/mm has an ectopic pregnancy unless she has had a recent complete spontaneous abortion between the time of the hCG and the sonogram.

In the absence of an intrauterine pregnancy, the most common sonographic signs of a tubal pregnancy include an adnexal mass (heterogeneous cystic and/or solid), a tubal ring (small round cyst with echogenic rim), a tubular mass consistent with a hematosalpinx, and echogenic free fluid in the cul-de-sac consistent with blood. Color Doppler often shows circumferential flow around a tubal ring, similar to a corpus luteum, but there is typically no flow inside a hematosalpinx. It is helpful to distinguish the ovary as separate from the ectopic pregnancy mass so as not to misdiagnose the mass as ovarian in origin. Pushing the mass gently past the ovary can demonstrate that they are separate.

Cornual ectopic pregnancies account for 1% to 6% of ectopic pregnancies and can be diagnosed if the gestational sac is located clearly outside of the endometrial cavity but surrounded by a thin rim of myometrium. If there is myometrium between the gestational sac and the endometrial cavity, then the pregnancy is cornual.

Cervical ectopic pregnancies account for less than 1% of ectopics and are diagnosed when the gestational sac is clearly seen within the cervix. A sac can slide through the cervix during a spontaneous pregnancy loss; therefore if this is suspected, a follow-up scan in 24 hours may be necessary to confirm a cervical pregnancy. Blood flow is typically present around the sac of a pregnancy implanted in the cervix as opposed to an ongoing pregnancy loss in which circumferential blood flow is absent.