[1] Univ North Carolina Chapel Hill, Gillings Sch Global Publ Hlth, Dept Maternal & Child Hlth, Chapel Hill, NC USA
[2] Univ North Carolina Chapel Hill, Carolina Populat Ctr, Chapel Hill, NC USA
[3] Univ North Carolina Chapel Hill, Gillings Sch Global Publ Hlth, Dept Epidemiol, 135 Dauer Dr,2101 McGavran Greenberg Hall,CB 7435, Chapel Hill, NC 27599 USA
[4] Univ North Carolina Chapel Hill, Cecil G Sheps Ctr Hlth Serv Res, Chapel Hill, NC USA
[5] Univ North Carolina Chapel Hill, Sch Med, Dept Obstet & Gynecol, Div Maternal Fetal Med, Chapel Hill, NC USA