Cystic Adenomyoma Mimicking a Cornual Heterotopic Pregnancy in a Patient With Concurrent Intrauterine Pregnancy: A Case Report

Cystic adenomyoma (CA) is a rare, focal form of adenomyosis that can mimic ectopic pregnancy and poses significant diagnostic challenges, particularly in early pregnancy. We describe a 47‐year‐old woman, gravida 3 para 2, with a previous caesarean section and a spontaneous, unintended pregnancy, who presented at six weeks of gestation with lower abdominal pain and nausea. Initial transvaginal ultrasound suggested a viable intrauterine pregnancy together with a heterogeneous right cornual mass measuring 24 × 25 mm, containing an internal hypoechoic component and showing incomplete peripheral vascularity, which raised suspicion for the cornual ectopic component of a heterotopic pregnancy. Because of this concern and the diagnostic uncertainty, diagnostic laparoscopy followed by mini‐laparotomy was undertaken. Intraoperatively, a firm cornual mass containing dark, old haemorrhagic material was identified and excised, and the intrauterine pregnancy was preserved. Histopathological examination revealed decidualised endometrium within adenomyomatous tissue, supporting the diagnosis of CA and excluding the suspected heterotopic pregnancy. Given the lesion’s location near the round‐ligament insertion, overlap with accessory cavitated uterine malformation (ACUM) is acknowledged. This case illustrates that CA can closely reproduce the imaging appearance of a cornual heterotopic pregnancy and should be considered in the differential diagnosis of an early‐pregnancy cornual mass, even in older reproductive‐age women. It also underscores the value of careful, multimodal imaging interpretation and, where feasible, adjunctive magnetic resonance imaging, together with a multidisciplinary approach involving radiologists, pathologists and gynaecological surgeons, to reduce the risk of misdiagnosis and to guide management when clinical history and imaging findings are ambiguous or conflicting.