[1] Univ Toronto, Fac Med, Dept Physiol, Toronto, ON M5S 1A8, Canada
[2] Univ Toronto, Fac Med, Dept Med, Toronto, ON M5S 1A8, Canada
[3] Univ Toronto, Fac Med, Dept Obstet & Gynaecol, Toronto, ON M5S 1A8, Canada
[4] Southampton Gen Hosp, MRC Epidemiol Resource Ctr, Southampton SO16 6YD, Hants, England