Step-by-step ultrasound guide for rudimentary horn pregnancy diagnosis.

Rudimentary horn pregnancy (RHP) is a rare form of ectopic pregnancy that poses significant risks if not recognised early, as rupture can result in severe maternal morbidity. Early diagnosis is both challenging and crucial. Ultrasound pattern recognition plays a central role in identifying RHP, with MRI serving as a valuable adjunct in suspicious cases. Here, we report a case of an unruptured RHP diagnosed in the early second trimester, following a failed medical termination attempt 6 days prior due to early fetal demise. We present a step-by-step ultrasound diagnostic approach, emphasising the importance of recognising this rare condition. In rare instances, as illustrated in this case, a large unruptured RHP may be misdiagnosed as an intrauterine pregnancy or another form of ectopic pregnancy. This underscores the need for clinicians to be familiar with atypical image findings. Accurate diagnosis and precise localisation of the pregnancy are essential for optimal management.