1. Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA,
2. Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
3. Department of Obstetrics and Gynecology, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
4. Department of Biostatistics, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
5. Department of Radiology, George Washington University Medical Center, Washington, DC, USA
6. Epidemiology Branch, NIEHS, Research Triangle Park, NC, USA