Familial Twinning: A Case for Superfetation in Man

Abstract

A family is described in which the tendency to bear twins is expressed in the offspring of males as well as females. All of the twins born in this family show marked discordance in birth weight and gestational age. In 5 of the 6 pairs, one twin was normal while the other was either a macerated fetal mass, stillborn, or died of prematurity in the neonatal period. The one pair in which both twins did survive is known to be DZ and showed a difference in maturity and a 21% discrepancy in birth weight. Thus, twinning in this family appears to be transmitted as an autosomal dominant trait which is expressed in the offspring of both female and male carriers. Of all possible genetic mechanisms which could explain this familial aggregation of markedly discordant twins, supcrfetation seems most consistent with the genetic transmission and expression of the trait in the offspring of both males and females. The most plausible explanation of the pedigree is that a dominant gene is segregating in the family which is expressed in the fetal placenta where it acts to reverse the normal hormonal inhibition of ovulation. Since both the father and the mother contribute to the genotype of the placenta, superfeiation could occur among offspring of both males and females who carry the gene.