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Hum. Reprod. Advance Access originally published online on September 25, 2006
Human Reproduction 2007 22(2):407-413; doi:10.1093/humrep/del374
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© The Author 2006. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Time to pregnancy and multiple births

R.M. Ferrari1, M.A. Cooney2, A. Vexler3, A. Liu3 and G.M. Buck Louis2,4

1 Department of Maternal and Child Health, University of North Carolina at Chapel Hill, Chapel Hill, NC 2 Epidemiology Branch and 3 Biometry Branch, National Institute of Child Health and Human Development, Rockville, MD, USA

4 To whom correspondence should be addressed at: Epidemiology Branch, National Institute of Child Health and Human Development, 6100 Executive Blvd., Room 7B03, Rockville, MD 20852, USA. E-mail: louisg{at}mail.nih.gov

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 ≤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.

Key words: fecundity/multiple births/time-to-pregnancy/twinning


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