[1] Duke Univ, Dept Obstet & Gynecol, Durham, NC 27707 USA
[2] Univ Calif San Francisco, Dept Obstet, Dept Gynecol, San Francisco, CA 94143 USA
[3] Univ Calif San Francisco, Dept Reprod Sci, San Francisco, CA 94143 USA
[4] Univ Calif San Francisco, Dept Epidemiol & Biostat, San Francisco, CA 94143 USA
[5] Univ Calif San Francisco, Med Effect Res Ctr Diverse Populat, San Francisco, CA 94143 USA