Live pregnancy in a non-communicating rudimentary horn with a coexisting ipsilateral ovarian cyst: the role of MRI in diagnosis and surgical planning

Pregnancy in a non-communicating rudimentary horn is a rare and potentially life-threatening form of ectopic gestation that arises in the setting of a Müllerian duct anomaly and is frequently diagnosed only after rupture. Pre-operative identification on ultrasonography alone is difficult because the findings can mimic a tubal ectopic pregnancy, an interstitial or angular pregnancy, or a pregnancy in a bicornuate uterus. We report a live pregnancy in a non-communicating rudimentary horn with a coexisting ipsilateral simple ovarian cyst, in which magnetic resonance imaging (MRI) established the diagnosis, distinguished the gestation from the adnexal lesion, and guided pre-operative surgical planning. A 25-year-old woman (gravida 2, one previous abortion) presented with two months of amenorrhoea, right lower abdominal pain and intermittent vaginal spotting. Transvaginal sonography (TVS) demonstrated a live extrauterine gestation in the right hemipelvis with an empty uterine cavity and an adjacent 4 cm right ovarian simple cyst, but could not reliably define the relationship between the gestation, the uterus and the adnexa. Multiparametric pelvic MRI characterised a left unicornuate uterus with a right non-communicating rudimentary horn (approximately 42 × 39 × 51 mm) containing a live gestation of about 8 weeks, and a separate right ovarian simple cyst; no communication with the native uterine cavity and no haemoperitoneum were identified. According to the American Society for Reproductive Medicine Müllerian Anomaly Classification 2021 (ASRM MAC 2021), the appearances corresponded to a unicornuate uterus with a non-communicating uterine remnant (rudimentary horn) containing functional endometrium, with a single cervix and a single vagina (ESHRE/ESGE equivalent, U4aC0V0). Elective laparotomy with excision of the rudimentary horn, ipsilateral salpingectomy and ovarian cystectomy (with ovarian conservation) was performed without complication. Histopathology confirmed products of conception within the rudimentary horn and a benign simple ovarian cyst. Pregnancy in a non-communicating rudimentary horn is an obstetric emergency because delayed recognition may lead to catastrophic uterine rupture. When sonography is inconclusive—particularly when coexisting adnexal pathology is present—MRI can provide precise anatomical delineation, support accurate classification of the Müllerian anomaly, differentiate the gestation from adjacent adnexal lesions and inform surgical planning. Early diagnosis and timely surgery remain the cornerstones of management.