[1] Columbia Univ, Dept Pathol, Coll Phys & Surg, New York, NY 10032 USA
[2] Univ Cape Town, Dept Obstet & Gynaecol, Cape Town, South Africa
[3] Columbia Univ, Gertrude Sergievsky Ctr, Coll Phys & Surg, New York, NY 10032 USA
[4] Columbia Univ, Dept Epidemiol, Mailman Sch Publ Hlth, New York, NY 10032 USA
[5] Columbia Univ, Mailman Sch Publ Hlth, Dept Biostat, New York, NY 10032 USA