[1] Johns Hopkins Med Inst, Dept Obstet & Gynecol, Baltimore, MD 21205 USA
[2] Johns Hopkins Med Inst, Dept Epidemiol, Baltimore, MD 21205 USA
[3] Johns Hopkins Med Inst, Dept Med, Baltimore, MD 21205 USA
[4] Johns Hopkins Med Inst, Dept Populat & Family & Reprod Hlth, Baltimore, MD 21205 USA