[1] Univ Washington, Dept Obstet & Gynecol, Seattle, WA 98195 USA
[2] Univ Washington, Dept Obstet & Gynecol, Seattle, WA 98195 USA
[3] Univ Washington, Dept Epidemiol, Seattle, WA 98195 USA
[4] Univ Washington, Dept Med, Seattle, WA 98195 USA
[5] Univ Nairobi, Dept Obstet & Gynecol, Nairobi, Kenya
[6] Univ Nairobi, Dept Med Microbiol, Nairobi, Kenya
[7] Univ British Columbia, Ctr Dis Control, Vancouver, BC V5Z 1M9, Canada