[1] Univ British Columbia, Dept Obstet, Vancouver, BC, Canada
[2] Univ British Columbia, Dept Gynecol, Vancouver, BC, Canada
[3] Univ British Columbia, Dept Pediat, Vancouver, BC, Canada
[4] British Columbia Womens Hosp & Hlth Ctr, Vancouver, BC, Canada
[5] Univ British Columbia, Sch Populat & Publ Hlth, Vancouver, BC, Canada
[6] Simon Fraser Univ, Dept Stat & Actuarial Sci, Burnaby, BC, Canada
[7] McGill Univ, Dept Epidemiol Biostat & Occupat Hlth, Montreal, PQ, Canada
[8] Univ British Columbia, Northern Med Program, Prince George, BC, Canada
[9] St Pauls Hosp, Providence Hlth Care, Ctr Hlth Evaluat & Outcome Sci, Vancouver, BC, Canada