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Am J Epidemiol. 2018 Sep 5. doi: 10.1093/aje/kwy188. [Epub ahead of print]

A Population-based Matched Sibling Analysis Estimating the Association between First Interpregnancy Interval and Birth Outcomes.

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School of Public Health, Curtin University, Bentley, Western Australia, Australia.
School of Nursing, Midwifery and Paramedicine, Curtin University, Bentley, Western Australia, Australia.
School of Public Health, Yale University, New Haven, Connecticut, United States.
School of Medicine, University of California San Francisco, San Francisco, California, United States.
London School of Hygiene and Tropical Medicine, London, United Kingdom.
School of Public Health, University of Sydney, Sydney, New South Wales, Australia.
Telethon Kids Institute, University of Western Australia, Subiaco, Western Australia, Australia.


The association between a single interpregnancy interval (IPI) on birth outcomes has not yet been explored using matched methods. We modelled the odds of preterm birth, small-for-gestational age and low birthweight in a second, liveborn infant in a cohort of 192,041 sibling pairs born in Western Australia between 1980 and 2010. The association between IPI and birth outcomes was estimated from the interaction between birth order and IPI (with 18-23 months as the reference category), using conditional logistic regression. Matched analysis showed the odds of preterm birth were higher for siblings born following an IPI <6 month (adjusted Interaction Odds Ratio [IOR]: 1.22; 95% CI: 1.06, 1.38) compared with 18-23 month IPI. There were no significant differences for IPI <6 months for other outcomes (small-for-gestational age or low birthweight). This is the first study to use matched analyses to investigate the association between a single IPI on birth outcomes. IPI <6 months were associated with increased odds of preterm birth in secondborn infants, although the association is likely smaller than previously estimated by unmatched studies.


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