Sirenomelia and monomelia with renal agenesis and amnion nodosum.

Interest has recently been directed towards sympodia, a foetal malformation better known by the synonyms sirenomelia, sympus dipus or mermaid foetus. This malformation consists of more or less complete fusion of the lower limbs to form a single extremity. Less frequently reported is the condition described by Ballantyne (1904) as monopodia, in which there is a single lower extremity which shows no indication that it has been derived from the fusion of two lower limbs. For this abnormality we prefer the term monomelia. Severe dysplasia of the foetal urinary tract is apparently an invariable accompaniment of sirenomelia and monomelia. Despite the considerable number of cases described in the literature scant attention has been paid to certain other associated features in the foetus. Only occasionally is reference made to the liquor amnii, and there are no previous histological studies of the placental membranes.