1. Department of Epidemiology, Harvard T. H. Chan School of Public Health, Boston, Massachusetts
2. Department of Family Practice, University of British Columbia, Vancouver, British Columbia, Canada
3. Department of Obstetrics and Gynaecology, University of British Columbia, Vancouver, British Columbia, Canada
4. Department of Biostatistics, Harvard T. H. Chan School of Public Health, Boston, Massachusetts
5. Department of Obstetrics and Gynecology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts