1. Center for Women's Health Research; University of North Carolina at Chapel Hill; 104B Market Street, Chapel Hill NC 27704 USA
2. Department of Epidemiology, Gillings School of Global Public Health; University of North Carolina at Chapel Hill; 135 Dauer Drive, Chapel Hill NC 27599-7400 USA
3. Department of Obstetrics and Gynecology, School of Medicine; University of North Carolina at Chapel Hill; 3009 Old Clinic Building, Chapel Hill NC 27599-7570 USA
4. Injury Prevention Research Center; University of North Carolina at Chapel Hill; CVS Plaza, Suite 500, 137 East Franklin Street, Chapel Hill NC 27599-7505 USA
5. Epidemico, Inc.; Boston MA USA