Study Design and Setting

We undertook a population-based cohort study in Ontario, Canada. Ontario has 140 000 births each year and is Canada’s most populous province.¹⁸ All prenatal and obstetric services delivered in and outside of hospitals are free of charge to Ontario residents. The current study included obstetric deliveries with a conception date between April 1, 2002, and March 31, 2012.

We obtained data from the Institute for Clinical Evaluative Sciences (ICES), a nonprofit, independent organization that holds data resulting from health care encounters of Ontario residents. ICES databases were linked using unique encoded identifiers and were analyzed at ICES. To identify study groups, we used the Canadian Institute for Health Information Discharge Abstract Database (hospitalizations), the Ontario Mental Health Reporting System (psychiatric hospitalizations), the National Ambulatory Care Reporting System (emergency department visits), the Ontario Health Insurance Plan database (primary care visits), and disability support information from the Ministry of Community and Social Services.¹⁹ The MOMBABY data set, which is derived from the Canadian Institute for Health Information Discharge Abstract Database, was used to identify obstetric deliveries and to estimate conception date based on gestational age at delivery.²⁰ The Registered Persons Database (birth date, postal code) and ICES databases described above were used to create outcomes and covariates.

Diagnostic codes follow the Canadian Coding Standards for the International Statistical Classification of Diseases and Related Health Problems 10th Revision for hospital databases and physician billing claim codes for the Ontario Health Insurance Plan database. Procedural codes follow the Canadian Classification of Health Interventions. Sociodemographic information, primary diagnosis, physician billing claims, and procedures are reliable and valid.²¹