Abstract
Background: Causal interpretation of associations between short interpregnancy interval, the duration from the preceeding birth to the conception of the next-born index child, and offspring’s psychological and educational problems may be influenced by a failure to account for unmeasured confounding.
Methods: Using population-based Swedish data from 1973-2009, we estimated the association between interpregnancy interval and outcomes (autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), severe mental illness, suicide attempt, criminality, substance use problem, and failing grades) while controlling for measured covariates. We then used cousin-comparisons, post-birth intervals (the interval between the second- and third-born siblings to predict second-born outcomes), and sibling-comparisons, to assess the influence of unmeasured confounding. We included an exploratory analysis of long interpregnancy interval.
Results: Interpregnancy intervals of 0-5 and 6-11 months were associated with higher odds of outcomes in cohort analyses. Magnitudes of association were attenuated following adjustment for measured covariates. Associations were eliminated for ADHD, severe mental illness, and failing grades, but maintained magnitude for ASD, suicide attempt, criminality, and substance use problem in cousin-comparisons. Post-birth interpregnancy interval and sibling-comparisons suggested some familial confounding. Associations did not persist across models of long interpregnancy interval.
Conclusions: Attenuation of the association in cousin-comparisons and comparable post-birth interval associations suggests that familial genetic or environmental confounding accounts for a majority of the association for ADHD, severe mental illness, and failing grades. Modest associations appear independent of covariates for ASD, suicide attempt, criminality, and substance use problem. Post-birth analyses and sibling-comparisons, however, show some confounding in these associations.
