The association between interpregnancy interval and severe maternal morbidities using revised national birth certificate data: A probabilistic bias analysis

Background Severe maternal morbidity continues to be on the rise in the US. Short birth spacing is a modifiable risk factor associated with maternal morbidity, yet few studies have examined this association, possibly due to few available data sources to examine these rare events. Objective To examine the association between interpregnancy interval (IPI) and severe maternal morbidity using near-national birth certificate data and account for known under-reporting using probabilistic bias analysis. Methods We used revised 2014-2017 birth certificate data, restricting to resident women with a non-first-born singleton birth. We examined the following: (a) maternal blood transfusion, (b) admission to intensive care unit (ICU), (c) uterine rupture (among women with a prior caesarean delivery) and (d) third- or fourth-degree perineal laceration (among vaginal deliveries) by IPI categories (<6, 6-11, 12-17, 18-23, 24-59 and 60+ months). Risk ratios and 95% confidence intervals were estimated using log-binomial regression, adjusting for select maternal characteristics. Probabilistic bias analyses were performed. Results Compared with IPI 18 to 23 months, adjusted models revealed that the risk of maternal transfusion followed a U-shaped curve with IPI, while risk of ICU admission and perineal laceration increased with longer IPI. Risk of uterine rupture was highest among IPI <6 months. With the exception of maternal transfusion, these findings persisted regardless of the extent or type of misclassification examined in bias analyses. Conclusions Associations between IPI and maternal morbidity varied by outcome, even after adjusting for misclassification of SMM. Differences across maternal health outcomes should be considered when counselling and making recommendations regarding optimal birth spacing.