1. Department of Obstetrics and Gynaecology University of British Columbia, and the Children's and Women's Hospital of British Columbia Vancouver British Columbia Canada
2. School of Population and Public Health University of British Columbia Vancouver British Columbia Canada
3. Reproductive Care Program of Nova Scotia Halifax Nova Scotia Canada
4. Centre for Surveillance and Applied Research, Public Health Agency of Canada, and the School of Epidemiology and Public Health University of Ottawa Ottawa Ontario Canada
5. Departments of Epidemiology and Occupation Health and of Pediatrics McGill University Montréal Quebec Canada