Sirenomelia (Mermaid syndrome) with bilateral renal agenesis and a single umbilical artery: An autopsy case report with embryological insights

Abstract

Sirenomelia, also known as mermaid syndrome, is a rare and typically lethal congenital anomaly characterized by the fusion of the lower extremities and multiple visceral malformations. Its pathogenesis remains debated, with proposed mechanisms including abnormal vascular development (vitelline artery steal) and defective caudal mesoderm formation (defective blastogenesis). We describe a full-term female fetus (gestational age of 37+1 weeks) born to a 21-year-old primigravida, in whom prenatal ultrasonography revealed growth restriction, anhydramnios, and bilateral renal agenesis. At delivery, the neonate showed fused lower limbs, imperforate anus, absent external genitalia, dysmorphic facial features consistent with Potter’s facies, and a two-vessel umbilical cord, and died two hours after birth. Autopsy confirmed bilateral renal agenesis, pulmonary hypoplasia, an absent urinary tract, and multiple dysmorphic features. Placental examination revealed delayed villous maturation with a single umbilical artery, and histology revealed lung immaturity inconsistent with gestational age. This case illustrates the classical presentation of sirenomelia associated with the Potter sequence and underscores the role of autopsy in clarifying its anomaly spectrum, supporting embryological hypotheses, and guiding prenatal counseling.