1. Department of Obstetrics and Gynecology, University of North Carolina, Chapel Hill
2. Department of Biostatistics, University of North Carolina, Chapel Hill
3. Department of Obstetrics and Gynecology and Preventive Medicine, University of Southern California, Los Angeles
4. National Institute for Occupational Safety and Health, Cincinnati, Ohio
5. Department of Statistical Science, Duke University, Durham, North Carolina
6. Epidemiology Branch, National Institute of Environmental Health Science/National Institutes of Health, Research Triangle Park, North Carolina