[1] Johns Hopkins Univ, Dept Populat & Family Hlth Sci, Baltimore, MD 21205 USA
[2] Brigham Young Univ, Dept Stat, Provo, UT 84602 USA
[3] Johns Hopkins Med Inst, Div Maternal Fetal Med, Baltimore, MD 21205 USA
[4] Johns Hopkins Univ, Dept Gynecol & Obstet, Baltimore, MD 21205 USA