[1] NICHHD, Div Epidemiol Stat & Prevent Res, US Dept HHS, Bethesda, MD 20892 USA
[2] Beth Israel Deaconess Med Ctr, Dept Med, Boston, MA 02215 USA
[3] Beth Israel Deaconess Med Ctr, Dept Obstet, Boston, MA 02215 USA
[4] Beth Israel Deaconess Med Ctr, Dept Gynecol, Boston, MA 02215 USA
[5] Harvard Univ, Sch Med, Boston, MA 02115 USA