September 25th, 2023

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Mechanisms of Twinning: IV. Sex Preference and Lactation
Authors:  Gary Steinman, M.D., Ph.D.
  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. (J Reprod Med 2001;46:1003-1007)
twin studies, twins, lactation
Keywords:  twin studies, twins, lactation
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