[1] Univ North Carolina, Sch Med, Chapel Hill, NC 27599 USA
[2] Univ South Carolina, Arnold Sch Publ Hlth, Dept Hlth Promot Educ & Behav, Columbia, SC USA
[3] Univ North Carolina, Gillings Sch Global Publ Hlth, Dept Epidemiol, Chapel Hill, NC USA
[4] Univ North Carolina, Sch Med, Dept Obstet & Gynecol, Chapel Hill, NC USA
[5] Univ North Carolina, Sch Med, Dept Pediat, 231 McNider Bldg CB 7225 333 S Columbia St, Chapel Hill, NC 27599 USA