title: Time to pregnancy and multiple births.
abstract: BACKGROUND
  Mothers of multiples are alleged to be more fecund than mothers of singletons. Some authors have suggested monitoring twinning rates for assessing temporal changes in a population's reproductive health.
  
  
  METHODS
  Using a nested case-control design, we estimated the odds of a multiple birth in relation to fecundity in the US Collaborative Perinatal Project inclusive of 8546 pregnant women who reported a known time-to-pregnancy (TTP) upon enrolment in the cohort, 1959-1966. Case mothers comprised 81 women giving birth to twins/triplets; control mothers comprised 243 women giving birth to singletons matched to case mothers on maternal age at a ratio of 3:1. The odds ratio (OR) for a multiple birth within 6 months of trying adjusting for maternal age and prior pregnancies was estimated using logistic regression. Discrete time Cox regression analysis was also utilized to estimate the fecundability OR.
  
  
  RESULTS
  Women with a TTP of <or=6 months were more likely to have a multiple birth than women reporting a TTP of >6 months [OR=1.95; 95% confidence interval (95% CI)=1.09-3.51]. Excluding pregnancies after 13+ months resulted in a loss of precision (OR=2.14; 95% CI=0.90-5.04).
  
  
  CONCLUSIONS
  These data support higher fecundity among mothers of multiples than mothers of singletons.
authors.affiliation: Department of Maternal and Child Health, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA.
authors.forename: R M
authors.initials: RM
authors.lastname: Ferrari
authors.forename: M A
authors.initials: MA
authors.lastname: Cooney
authors.forename: A
authors.initials: A
authors.lastname: Vexler
authors.forename: A
authors.initials: A
authors.lastname: Liu
authors.forename: G M
authors.initials: GM
authors.lastname: Buck Louis
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D005260:Female; D005298:Fertility; D006801:Humans; D008423:Maternal Age; D016017:Odds Ratio; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000706:statistics & numerical data*