455Association of interpregnancy interval and preterm births: what does a sibling-matched study indicate?

Background Most evidence for interpregnancy interval (IPI) and adverse birth outcomes come from between-women (unmatched) studies that are prone to incomplete control for confounders that vary between women. Comparing pregnancies to the same women (sibling-matched) can address this issue. Methods We conducted an international longitudinal study of births from Australia, Finland, Norway, and United States (California) covering over three decades (1980-2017). We included 5,523,914 births to 3,849,737 women, and within each country, we first investigated the associations between IPI and preterm birth (PTB), spontaneous PTB using logistic regression (unmatched analyses). Second, we used conditional logistic regression comparing IPIs in the same women (sibling-matched analyses), with 2,908,907 births to 1,234,730 women having at least two IPIs. Random effects meta-analysis was used to calculate pooled effect estimates. Results Compared to an IPI of 18-23 months, there was insufficient evidence of association between IPI