Symmelia in one of monozygotic twins

Postmortem dissection of a liveborn infant with symmelia who was one of a pair of monozygotic female twins revealed fusion of the preaxial halves of the hindlimbs to form a single hip joint, a femur with two medial and no lateral condyles, and knee and ankle joints that flexed backwards. The kidneys, bladder, uterus, and rectum were absent. Twenty‐three instances of twinning were found among 327 cases of symmelia reported in the medical literature. Only two pairs were definitely concordant, but whenever data were given they supported monozygosity. There appears to be more than a 100‐fold increase in the incidence of symmelia in monozygotic twin births as compared with single births. The high frequency of discordance could result from unequal expression of a cytoplasmic abnormality. Alternately, the developmental arrest of the primitive streak that leads to symmelia may also initiate the formation of a second primitive streak, thereby giving rise to twin embryos.