Defective blastogenesis of postnatally diagnosed type VI sirenomelia in a young primigravida: A case report.

Sirenomelia is a rare lethal multi-systemic birth malformation in which the two lower limbs are replaced with a rotated single midline tail-like limb. Several hypotheses try to explain this syndrome, with the most prominent theories being the "vascular steal hypothesis" and the "defective blastogenesis hypothesis." We report a case of a baby with sirenomelia who had a single femur and a single tibia, which classify the case as type VI on Stocker and Heifetz classification. The only risk factor in our case is young maternal age. The baby had a single umbilical artery, a prominent feature of the vascular steal hypothesis. Nonetheless, it also had upper limb deformity, which can be better explained by the defective blastogenesis hypothesis. Our case supports the defective blastogenesis theory of sirenomelia more than the vascular steal hypothesis as it has both a single umbilical artery and upper limb deformity. Also, our case serves as a teaching lesson that indicates the importance of an obstetric ultrasound before a cesarean section has to be done to avoid unnecessary surgery for life incompatible congenital anomaly.