Importance — Severe maternal morbidity (SMM) is defined by potentially life-threatening conditions. The association between the number of SMM indicators and maternal death is not known.
Objective — To quantify the relation between the number of SMM indicators and maternal mortality.
Design, Setting, and Participants — Retrospective population-based study of all livebirth and stillbirth deliveries from 20 weeks’ gestation onward, in the province of Ontario, Canada, 2002-2017.
Exposure: Number of SMM identified between 20 weeks’ gestation and 42 days postpartum.
Main Outcomes and Measures — Maternal death occurring from delivery up to 42 days postpartum.
Results — Among 1,953,943 births there were 181 maternal deaths -- a rate of 9.3 per 100,000 births. Among these 181 deaths, 68% had at least 1 SMM indicator, compared to just 1.7% of survivors. Women who died tended to be older, nulliparous, of lower income and Afro-Caribbean origin, with a multi-fetal pregnancy, pre-existing diabetes, hypertension and renal disease. Stillbirth was much more common among maternal deaths (17.1%) than in survivors (0.6%). The rate of maternal mortality rose exponentially with the number of SMM: 0 (3.0 per 100,000 births), 1 (71.7 per 100,000 births), 2 (385.9 per 100,000 births), 3 (1274.2 per 100,000 births), 4 (2236.8 per 100,000 births), 5 (4285.7 per 100,000 births) and ≥ 6 indicators (9422.5 per 100,000 births). Adjusted RRs for maternal death ranged from 20.1 (95% CI 11.6-34.7) with 1 SMM, to 2192.0 (95% CI 1287.0-3735.0) with ≥ 6 SMM, compared to 0 SMM.
Conclusions and Relevance — Maternal death is exponentially associated with the number of SMM indicators, and occurs in certain identifiable groups of women. Targeting preventable SMM, or limiting its progression once realized, could lead to a reduction in maternal deaths.