We examined the associations between interpregnancy interval, the duration from the preceding birth to the conception of the next-born index child, and adverse birth outcomes using designs that adjust for measured and unmeasured factors.METHODS:
We used population-based Swedish registries to study a prospective cohort of children born 1973 to 2009 (n=1,418,820) to estimate the associations between interpregnancy interval and adverse birth outcomes. Analyses included cousin- and sibling-comparisons and postbirth intervals (i.e., the interval between second- and third-born offspring predicting second-born outcomes) to address unmeasured familial confounding.RESULTS:
Traditional cohort-wide analyses showed higher odds of preterm birth (PTB) [adjusted odds ratio (aOR) 1.51, 99% CI 1.39-1.63] and low birth weight (LBW; aOR 1.25, 99% CI 1.13-1.39) after short interpregnancy interval (0 to 5 months) compared to offspring born after an interpregnancy interval of 18 to 23 months. Except for PTB (aOR 1.72, 99% CI 1.26-2.35), associations were attenuated in cousin- and sibling-comparisons. For interpregnancy intervals of 60 months or more, odds of PTB (aOR 1.51, 99% CI 1.43-1.60), LBW (aOR 1.61, 99% CI 1.50-1.73), and SGA (aOR 1.54, 99% CI=1.42-1.66) were higher when compared with the reference interval. Associations between long interpregnancy interval and adverse birth outcomes, however, remained through cousin- and sibling-comparisons. Postbirth interval analyses showed familial confounding is present for short interpregnancy intervals, but supported independent associations for long interpregnancy intervals.CONCLUSION:
Familial confounding explains most of the association between short interpregnancy interval and adverse birth outcomes, while associations with long interpregnancy intervals were independent of measured and unmeasured factors.