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Hum. Reprod. Advance Access published online on September 25, 2006
Human Reproduction, 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
Received May 26, 2006
Revised August 18, 2006
Accepted August 25, 2006

Article

Time to pregnancy and multiple births

R.M. Ferrari 1, M.A. Cooney 2, A. Vexler 3, A. Liu 3, and G.M. Buck Louis 2 *

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

* To whom correspondence should be addressed.
G.M. Buck Louis, E-mail: louisg{at}mail.nih.gov


  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 ≤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.

Keywords: fecundity/multiple births/time-to-pregnancy/twinning.
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