title: Heritability of parturition timing: an extended twin design analysis.
abstract: OBJECTIVE
  The objective of the study was to assess relative maternal and paternal genetic influences on birth timing.
  
  
  METHODS
  Utilizing The Netherlands Twin Registry, we examined the correlation in birth timing of infants born to monozygotic (MZ) twins and their first-degree relatives (dizygotic twins and siblings of twins). Genetic models estimated the relative influence of genetic and common environmental factors through model fitting of additive genetic (A), common environmental (C), individual-specific environmental factors, and combinations thereof.
  
  
  RESULTS
  We evaluated birth timing correlation among the infants of 1390 twins and their 644 siblings. The correlation in MZ female twins (r = 0.330) was greater than MZ male twins (r = -0.096). Positive correlation were also found in sister-sister pairs (r = 0.223) but not in brother-brother (r = -0.045) or brother-sister pairs (r = -0.038). The most parsimonious AE model indicated a significant maternal contribution of genetic and individual-specific environmental factors to birth timing, but no paternal heritability was demonstrated. Heritability of birth timing in women was 34%; and the remaining variance (66%) was caused by individual-specific environmental factors.
  
  
  CONCLUSIONS
  Our data implicate a significant contribution of maternal but not paternal genetic influences on birth timing.
authors.affiliation: Department of Pediatrics, Washington University, St Louis, MO, USA.
authors.forename: Zachary A-F
authors.initials: ZA
authors.lastname: Kistka
authors.forename: Emily A
authors.initials: EA
authors.lastname: DeFranco
authors.forename: Lannie
authors.initials: L
authors.lastname: Ligthart
authors.forename: Gonneke
authors.initials: G
authors.lastname: Willemsen
authors.forename: Jevon
authors.initials: J
authors.lastname: Plunkett
authors.forename: Louis J
authors.initials: LJ
authors.lastname: Muglia
authors.forename: Dorret I
authors.initials: DI
authors.lastname: Boomsma
mesh_terms: D000328:Adult; D004777:Environment; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009426:Netherlands; D036801:Parturition / Q000235:genetics*; D012737:Sex Factors; D013997:Time Factors; D014427:Twins / Q000235:genetics*; D014429:Twins, Dizygotic / Q000235:genetics; D014430:Twins, Monozygotic / Q000235:genetics