[1] Johns Hopkins Med, Dept Obstet & Gynecol, Baltimore, MD 21287 USA
[2] Univ Iowa, Med Ctr, Dept Obstet & Gynecol, Iowa City, IA USA
[3] Wake Forest Univ, Sch Med, Dept Obstet & Gynecol, Winston Salem, NC 27109 USA
[4] Harvard Univ, Brigham & Womens Hosp, Sch Med, Dept Obstet & Gynecol, Boston, MA 02115 USA