A Population-Based Matched-Sibling Analysis Estimating the Associations Between First Interpregnancy Interval and Birth Outcomes

Abstract

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.

Developing evidence-based recommendations for optimal interpregnancy intervals in high-income countries: protocol for an international cohort study

“…Matched analyses will be undertaken to address secondary aims, where possible. In circumstances where within-mother matching is not feasible or not possible (eg, subpopulations with small sample sizes which would be further restricted by requiring more than two consecutive pregnancies per mother), unmatched models or alternative matching methods will be used (eg, propensity score stratification; sibling-matched methods using a single IPI) 8 44 45…”

Beyond birth outcomes: Interpregnancy interval and injury‐related infant mortality

“…11 However, recent analysis using matched designs has cast doubt on the causal effect of short IPI on adverse outcomes, arguing that previous associations have been confounded by factors such as socio-economic status (SES). [12][13][14] A recent series of articles on birth spacing published in this journal discussed the strengths and limitations of these approaches and called for an examination of additional outcomes, beyond the perinatal period, to guide evidence-based recommendations. 15,16 Limited data exist on the association between short IPI and external causes of infant death, which may operate through causal mechanisms outside of maternal nutritional depletion, such as divided parental time and resources described above.…”

“…The most common explanation for these associations is the ‘maternal depletion hypothesis’, which posits that adverse birth outcomes associated with short IPI are the result of depleted maternal nutritional stores due to insufficient recuperation time between pregnancies . However, recent analysis using matched designs has cast doubt on the causal effect of short IPI on adverse outcomes, arguing that previous associations have been confounded by factors such as socio‐economic status (SES) . A recent series of articles on birth spacing published in this journal discussed the strengths and limitations of these approaches and called for an examination of additional outcomes, beyond the perinatal period, to guide evidence‐based recommendations .…”

Are sibling models a suitable tool in analyses of how reproductive factors affect child mortality?

“…Many researchers have therefore estimated sibling models, and thus controlled for unobserved parental and environmental characteristics that are shared by the siblings, although it is a disadvantage that one-child families and families where all children have had the same outcomes do not contribute in the estimation. For example, sibling models have been used in several recent studies of the importance of birth intervals for the chance of preterm birth or low birth weight (Ball et al 2014;Shachar et al 2016;Hanley et al 2017;Class et al 2017;Regan et al 2018), child mortality (Molitoris 2018;Molitoris, Barclay, and Kolk 2019), or socioeconomic outcomes (Barclay and Kolk 2017).…”