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).