1. Department of Obstetrics and Gynecology, University of North Carolina, Chapel Hill, NC, USA
2. Department of Biostatistics, University of North Carolina, Chapel Hill, NC, USA
3. Department of Biostatistics & Carolina Population Center, University of North Carolina, Chapel Hill, NC, USA
4. Division of Applied Research and Technology, National Institute for Occupational Safety and Health, Cincinnati, OH, USA
5. Epidemiology Branch, National Institute of Environmental Health Science/National Institutes of Health, Research Triangle Park, NC, USA