Mechanisms of twinning: X. The male factor

To evaluate the possibility of male-line inheritance of dizygotic twinning potential.

Women with multiple births who had previously volunteered to participate in studies in this series were asked about the frequency of males genetically related to the index mother's husband who had also fathered multiples. Only spontaneous gestations were considered.

If male involvement in twin pregnancies were entirely random, <1% would have had male relatives who had also fathered multifetal gestations. However, 30% were found to have such relatives in the families evaluated here.

These results suggest that the capability to father multiples (twins or higher) spontaneously is, in many cases, an inherited trait along the male line--i.e., the male factor in twinning. Producing multiples requires not only women able to doubly ovulate but also men capable of supplying active sperm that could fertilize >1 egg in a given cycle.