Mechanisms of twinning. IV. Sex preference and lactation.

OBJECTIVE

To examine the likelihood of twinning through conception during lactation and to attempt to explain why there is an increased sex ratio with monozygotic twinning.

STUDY DESIGN

Parturients who had become pregnant while still breastfeeding were asked to supply information about their gestations. Also, records from two maternity services over a three-year period were examined for the sex of monozygotic twin sets. Zygosity was confirmed with the pathology reports of the placentas.

RESULTS

Much like that in a previously published study, the sex ratio (females/total) was found to be 0.60 in monozygotic twins. In the case of lactating gravidas, the incidence of twinning was much higher than in a random population (11.4% versus 1.1%).

CONCLUSION

These findings are consistent with the model of twinning as a consequence of a depressed calcium environment and its effect on embryonic intercellular bonding.