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Background
Ectopic pregnancy occurs when the blastocyst implants outside the uterine cavity (1.5–2.0% of all pregnancies). Majority (~95%) are tubal, involving the isthmic, ampullary, and fimbrial segments. Unusual sites (~5%) include the cervix, fallopian tube's interstitial segment, cesarean scars, myometrium, ovaries, and peritoneal cavity. Rare forms of ectopic pregnancy include heterotopic and twin ectopic pregnancies. They are associated with serious and potentially life-threatening complications. Hence, accurate diagnosis is vital to ensure such patients receive timely treatment.

Educational Goals / Teaching Points
We will discuss the risk factors, distinctive sonographic features, pearls, and pitfalls in the diagnosis of ectopic pregnancies.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Key sonographic features of various presentations of ectopic pregnancies. - Interstitial pregnancy: eccentric gestational sac and thin surrounding myometrium. Interstitial line connecting the endometrial cavity to the interstitial mass/gestational sac (represents the interstitial fallopian tube). - Cervical pregnancy: gestational sac below the internal os with a fetal heartbeat. - Cesarean scar pregnancy: implantation at the scar site. Thinning of the myometrium and an anterior lower uterine segment gestational sac. - Intramural pregnancy: implantation into the uterine myometrium. - Ovarian pregnancy: gestational sac with a thick, hyperechoic rim on or within the ovarian parenchyma. - Heterotopic pregnancy: has both intra- and extrauterine gestational sacs and may have two adnexal heartbeats in twin ectopic pregnancies.

Conclusion
Pearls and pitfalls: accurate diagnosis of uncommon ectopic pregnancies is vital as misdiagnosis leads to mismanagement, which can cause life-threatening outcomes. - Interstitial pregnancy: when rupture occurs, the proximity of the gestational sac to the myometrial vasculature can result in profuse life-threatening complications (mortality rate 15 times higher than in other tubal ectopics) - Cervical pregnancy: dilation and curettage (D&C) in a cervical ectopic pregnancy (vs an intrauterine pregnancy) can lead to life-threatening bleeding due to trophoblastic invasion of the cervix and the absence of muscle tissue in the cervical wall to assist in hemostasis. - Cesarean scar pregnancy: many patients (up to 40%) may not display specific clinical symptoms; hence, careful sonographic evaluation of the scar site is vital. - Heterotopic pregnancy: if the intrauterine pregnancy is missed, treating the ectopic with methotrexate will risk the viability of the intrauterine pregnancy.