1. Departments of Epidemiology (M.R.S., H.Z., D.M., S.D.H.), School of Public Health, University of Michigan, Ann Arbor, Michigan 48109
2. Biostatistics (B.N.), School of Public Health, University of Michigan, Ann Arbor, Michigan 48109
3. Department of Obstetrics and Gynecology (J.F.R.), University of Michigan Health System, Ann Arbor, Michigan 48109