Sirenomelia sequence associates a fusion of inferior legs with renal anomalies until bilateral agenesis. It is a rare and lethal polymalformation. The purpose of the ultrasonographic study is to identify the sirenomelia as early as possible during pregnancy and to differentiate it from caudal regression syndrome. A case of sirenomelia diagnosed early is reported together with a review of the literature. The ultrasonographic diagnosis, associated defects, the interest of color Doppler study of abdominal vasculature are discussed. Antenatal ultrasonographic diagnosis should be obtained as early as possible, before 20th gestational week at the latest. Color Doppler is helpful to confirm the diagnosis in case of bilateral renal agenesis. The main differences between sirenomelia and caudal regression syndrome (which requires a very different genetic counselling) are summarized in a table.