[1] Johns Hopkins Univ, Sch Med, Dept Med, Div Gen Internal Med, Baltimore, MD 21205 USA
[2] Johns Hopkins Univ, Sch Publ Hlth, Dept Populat Family & Reprod Hlth, Baltimore, MD USA
[3] Johns Hopkins Univ, Sch Med, Dept Gynecol & Obstet, Baltimore, MD 21205 USA
[4] Univ N Carolina, Sch Med, Dept Obstet & Ginecol, Chapel Hill, NC USA
[5] Johns Hopkins Univ, Sch Publ Hlth, Dept Epidemiol, Baltimore, MD 21205 USA