[1] Univ British Columbia, Dept Obstet, Vancouver, BC, Canada
[2] Univ British Columbia, Dept Gynaecol, Vancouver, BC V5Z 1M9, Canada
[3] Univ British Columbia, Dept Hlth Care & Epidemiol, Vancouver, BC V6T 1W5, Canada
[4] Univ British Columbia, Dept Med, Vancouver, BC, Canada
[5] Univ British Columbia, UBC Ctr Dis Control, Vancouver, BC V5Z 1M9, Canada