美人鱼综合症(英文：Sirenomelia 或 Mermaid Syndrome)是一种非常罕见的先天性缺陷，两条腿融合在一起，看起来像“美人鱼”。估计在大约十万个新生儿中会有一例(与连体双胞胎几率相当)。婴儿通常因为并发肾脏和膀胱异常，会在出生后一两天内死亡。约有一半的美人鱼综合征病例都会胎死腹中，并且这种缺陷在同卵双胞胎中发生的几率要高于非双胞胎和异卵双胞胎的100倍。目前，利用外科手术可以将融合的下肢，或融合的内脏分离。实验证明骨形成蛋白-7(Bmp7)可能在此疾病的发病中起作用.

Sirenomelia, alternatively known as Mermaid Syndrome, is a rare congenital deformity in which the legs are fused together, giving them the appearance of a mermaid's tail.

This condition is found in approximately one out of every 100,000 live births (about as rare as conjoined twins) and is usually fatal within a day or two of birth because of complications associated with abnormal kidney and urinary bladder development and function. More than half the cases of sirenomelia result in stillbirth and this condition is 100 times more likely to occur in identical twins than in single births or fraternal twins. It results from a failure of normal vascular supply from the lower aorta in utero. Maternal diabetes has been associated with caudal regression syndrome and sirenomelia, although a few sources question this association.