title: Mechanisms of twinning. IV. Sex preference and lactation.
abstract: 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.
  
  
  METHODS
  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%).
  
  
  CONCLUSIONS
  These findings are consistent with the model of twinning as a consequence of a depressed calcium environment and its effect on embryonic intercellular bonding.
authors.affiliation: Department of Obstetrics and Gynecology, Long Island Jewish Medical Center, New Hyde Park, New York, USA. dav4601@aol.com
authors.forename: G
authors.initials: G
authors.lastname: Steinman
mesh_terms: D002118:Calcium / Q000502:physiology; D010064:Embryo Implantation / Q000502:physiology; D005260:Female; D005306:Fertilization / Q000502:physiology; D006801:Humans; D007774:Lactation / Q000502:physiology*; D008297:Male; D011247:Pregnancy; D011446:Prospective Studies; D012744:Sex Ratio; D013997:Time Factors; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic
chemical_list: D002118:Calcium