Time to pregnancy and multiple births

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. Resutls: Women with a TTP of ≤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.