[1] Univ Calif San Francisco, Dept Family & Community Med, San Francisco, CA 94143 USA
[2] Univ Calif San Francisco, Dept Obstet Gynecol & Reprod Sci, San Francisco, CA USA
[3] Kaiser Permanente Ctr Hlth Res, Portland, OR USA
[4] Columbia Univ, Dept Obstet & Gynecol, Med Ctr, New York, NY USA