1. Department of Epidemiology, School of Public Health, University of North Carolina-Chapel Hill, Chapel Hill, North Carolina.
2. Carolina Population Center, University of North Carolina-Chapel Hill, Chapel Hill, North Carolina.
3. Department of Biostatistics, School of Public Health, University of North Carolina-Chapel Hill, Chapel Hill, North Carolina.
4. Department of Obstetrics and Gynecology, School of Medicine, University of North Carolina-Chapel Hill, Chapel Hill, NC.