Risk of adverse maternal outcomes among pregnancies with gestational diabetes mellitus in Ontario, Canada, 2012–2020: a retrospective cohort study

The rate of gestational diabetes mellitus (GDM) has increased over the past decades, but it’s unclear whether associations with maternal outcomes have changed. We aimed to describe rates of adverse maternal outcomes following deliveries with and without GDM over time and assess risks in these outcomes for GDM by delivery period. This population-based retrospective cohort study was conducted using provincial birth registry linked with health administrative databases in Ontario, Canada. All singleton hospital deliveries between April 1, 2012 and March 31, 2020 were included. We assessed the trends of adverse maternal outcomes among GDM and non-GDM pregnancies and used modified Poisson regression to estimate associations between GDM and adverse maternal outcomes, using crude and adjusted relative risk (aRR) and risk difference (aRD) with 95% confidence intervals (CIs). Outcome measures included labour induction, caesarean section (CS), assisted vaginal delivery, gestational hypertension/preeclampsia, and maternal morbidity or mortality. Among 1 044 258 deliveries, 82 896 (7.9%) had GDM. The age-adjusted rate of GDM increased from 6.2 (95% CI 6.1–6.4) to 10.2 (95% CI 10.0-10.4) per 100 deliveries from fiscal year 2012/13 to 2019/20. Overall, GDM were at a higher risk (aRR [95% CI]) of induction (1.61 [1.59, 1.62]), CS (1.08 [1.06, 1.09]) and gestational hypertension/preeclampsia (1.35 [1.32, 1.38]). The risk of gestational hypertension/preeclampsia for GDM, compared to no GDM, attenuated from an aRR of 1.45 (1.41, 1.49) in 2012/13-2015/16 to an aRR of 1.29 (1.25, 1.32) in 2016/17-2019/20. The strength of the association between GDM and induction (1.62 [1.60, 1.64] vs. 1.60 [1.59, 1.62]), CS (1.10 [1.08, 1,12] vs. 1.07 [1.05, 1.08]), assisted vaginal delivery (0.96 [0.92, 1.00] vs. 0.94 [0.90, 0.98]), and maternal morbidity and mortality (0.93 [0.78, 1.08] vs. 1.09 [0.97, 1.20]) remained stable over time. In this large population-based study of singleton hospital deliveries in Ontario, Canada, GDM was associated with higher risks of certain maternal adverse outcomes; however, these risks did not increase despite the increasing rate of GDM over the 8-year period, except for postpartum hemorrhage with interventions. Future large prospective studies should prioritize investigation into the risks of maternal outcomes across different glycemic diagnostic thresholds to inform cost-effective health care resource allocation for GDM pregnancies.