AbstractWe present a case of sirenomelia diagnosed on a first trimester ultrasound at 10 weeks' gestation and confirmed on 3D‐ultrasound and MRI. The pregnancy was terminated at 15 gestational weeks and the post‐mortem examination, including RX and microscopy, is presented. The sirenomelia sequence is a rare and lethal anomaly characterized by fusion, rotation, hypotrophy or atrophy of the lower limbs and severe urogenital abnormalities leading to oligohydramnios in the second half of pregnancy. Our case illustrates that the diagnosis of sirenomelia can be reliably made in the first trimester. Copyright © 2006 John Wiley & Sons, Ltd.