title: The correlation in birth timing between singleton and twin gestations in the same mother.
abstract: OBJECTIVE
  To test the hypothesis that the timing of twin births is correlated to that of their singleton siblings.
  
  
  METHODS
  Using the Missouri Department of Health's birth certificate database we performed a retrospective population-based cohort study of 2222 women who had at least one singleton and one twin birth (1989-1997). Pearson correlation analysis was conducted to examine the relationship in gestational age between singleton and twin births occurring in the same mother.
  
  
  RESULTS
  Among 2222 mothers who had both a singleton and twin pregnancy during the study period, the mean difference in birth timing between singleton and twin siblings was 3.05 weeks (+/-2.9 weeks). The correlation between singleton and twin birth timing was statistically significant (r = 0.28, p < 0.001), and remained significant even after controlling for potential confounders (beta = 0.459, p < 0.001).
  
  
  CONCLUSIONS
  We identified a significant correlation in the timing of birth between singleton and twin pregnancies in the same mother, which could be due to shared genetic, environmental, medical, social and other influences. This finding could be useful to help predict birth timing in mothers at especially high risk of preterm birth, those with a twin gestation.
authors.affiliation: Department of Obstetrics and Gynecology, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267-0526, USA. emily.defranco@uc.edu
authors.forename: Emily A
authors.initials: EA
authors.lastname: Defranco
authors.forename: Jen Jen
authors.initials: JJ
authors.lastname: Chang
authors.forename: George A
authors.initials: GA
authors.lastname: Macones
authors.forename: Louis J
authors.initials: LJ
authors.lastname: Muglia
mesh_terms: D000328:Adult; D015331:Cohort Studies; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D016014:Linear Models; D008423:Maternal Age; D036801:Parturition; D011247:Pregnancy; D011272:Pregnancy, Multiple; D047928:Premature Birth; D012189:Retrospective Studies; D035781:Siblings; D014427:Twins