First trimester diagnosis of sirenomelia: a case report and review of the literature.

Sirenomelia sequence is a rare lethal pattern of congenital anomalies characterized by a number of hallmark skeletal anomalies, including fusion of the lower extremities or a single lower limb, bilateral renal agenesis or dysgenesis with absent or hypoplastic renal arteries, oligohydramnios, and the presence of aberrant vasculature. The etiology is still controversial. Prognosis is very poor, with the babies being stillborn or succumbing soon after birth. In the second trimester, oligohydramnios due to renal agenesis makes the diagnosis of sirenomelia difficult. Conversely, in the first trimester, the amniotic fluid volume is usually normal, unrelated to the fetal urine production. Therefore, a first-trimester or early second trimester anatomic survey of the fetus is proposed as preferable and more accurate for the diagnosis of this rare anomaly. In this article, we report a case of sirenomelia detected by two- and three-dimensional ultrasound in the 11th week of gestation and the associated literature is discussed.