A 29-year-old woman with six weeks of amenorrhoea and acute right iliac pain has a positive pregnancy test. Ultrasound shows an eccentric gestational sac high in the fundus lateral to the endometrial cavity, surrounded by a thin rim of myometrium measuring less than 5 mm. The most likely diagnosis is:

• A Abdominal pregnancy
• B Angular intrauterine pregnancy
• C Ovarian ectopic pregnancy
• D Cornual (interstitial) ectopic pregnancy ✓

Explanation

An interstitial pregnancy lies within the interstitial portion of the fallopian tube as it traverses the myometrium. Ultrasound shows an eccentric sac separated from the endometrial cavity with a thin surrounding myometrial mantle, classically less than 5 mm. Because the myometrium around it is distensible and richly supplied by the uterine and ovarian arterial anastomosis, rupture occurs late and causes catastrophic haemorrhage. An angular pregnancy is intrauterine and retains a thick complete myometrial covering.

Reference: Rumack's Diagnostic Ultrasound, 5th ed.

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Written and medically reviewed by the StethoPrep medical team.