1. Department of Epidemiology, School of Public Health, and Carolina Population Center, School of Medicine, University of North Carolina, Chapel Hill, NC, USA,
2. Carolina Population Center, School of Medicine, University of North Carolina, Chapel Hill, NC, USA,
3. Obstetrics and Gynecology, School of Medicine, University of North Carolina, Chapel Hill, NC, USA,
4. Obstetrics and Gynecology, School of Medicine, University of North Carolina, Chapel Hill, NC and Wake Area Health Education Center, Wake Medical Center, Raleigh, NC, USA,
5. Carolina Population Center, School of Medicine, University of North Carolina, Chapel Hill, NC and Wake Area Health Education Center, Wake Medical Center, Raleigh, NC, USA,
6. Obstetrics and Gynecology, School of Medicine, University of North Carolina, Chapel Hill, NC, USA