Population and data sources
All women with a pregnancy lasting beyond 20 weeks’ gestation, and who delivered within an Ontario hospital between April 1, 2006 and March 31, 2014, were identified within the Better Outcomes Registry & Network (BORN) databases.14 Data beyond 2014 were not available in these datasets. The BORN registry captures over 99% of hospital births in the province, and has been validated for data completeness and accuracy.15,16 We used the Registered Persons Database, the Immigration, Refugees and Citizenship Canada’s Permanent Resident Database, the Ontario Health Insurance Plan (OHIP) outpatient claims database, and the Canadian Institute for Health Information (CIHI) Discharge Abstract Database to capture maternal demographics, pre-existing health conditions and diagnoses and procedures documented during a hospitalization (Table S1 for variables and diagnostic codes used to develop the study cohort). The datasets were linked using unique encoded identifiers and analysed at ICES― a not-for-profit provincial research entity that houses a large network of health administrative databases (https://www.ices.on.ca/).
We excluded ectopic pregnancies, pregnancies resulting in abortion or miscarriage or ending before 20 weeks’ gestation. We randomly sampled one birth (live- or stillbirth) per woman to avoid potential correlations among pregnancies of multiparous women (Table S1; Figure S1).