Uncommon Implantation Sites of Ectopic Pregnancy: Thinking beyond the Complex Adnexal Mass

Ectopic pregnancy occurs when implantation of the blastocyst takes place in a site other than the endometrium of the uterine cavity. Uncommon implantation sites of ectopic pregnancy include the cervix, interstitial segment of the fallopian tube, scar from a prior cesarean delivery, uterine myometrium, ovary, and peritoneal cavity. Heterotopic and twin ectopic pregnancies are other rare manifestations. Ultrasonography (US) plays a central role in diagnosis of uncommon ectopic pregnancies. US features of an interstitial ectopic pregnancy include an echogenic interstitial line and abnormal bulging of the myometrial contour. A gestational sac that is located below the internal os of the cervix and that contains an embryo with a fetal heartbeat is indicative of a cervical ectopic pregnancy. In a cesarean scar ectopic pregnancy, the gestational sac is implanted in the anterior lower uterine segment at the site of the cesarean scar, with thinning of the myometrium seen anterior to the gestational sac. An intramural gestational sac implants in the uterine myometrium, separate from the uterine cavity and fallopian tubes. In an ovarian ectopic pregnancy, a gestational sac with a thick hyperechoic circumferential rim is located in or on the ovarian parenchyma. An intraperitoneal gestational sac is present in an abdominal ectopic pregnancy. Intra- and extrauterine gestational sacs are seen in a heterotopic pregnancy. Two adnexal heartbeats suggest a live twin ectopic pregnancy. Recognition of the specific US features will help radiologists diagnose these uncommon types of ectopic pregnancy.