1. Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina.
2. Department of Obstetrics and Gynecology, University of North Carolina at Chapel Hill, North Carolina.
3. Department of Biostatistics, University of North Carolina at Chapel Hill, North Carolina.
4. Social and Scientific Systems, Inc., Durham, North Carolina.
5. National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina.