1. Department of Epidemiology (M.F.R.S., D.M., M.Y., M.L.J., S.H.), School of Public Health, University of Michigan 48109
2. Department of Obstetrics and Gynecology (M.F.R.S., A.D.E., J.F.R.), University of Michigan Health Sciences System, Ann Arbor, Michigan 48109
3. Department of Statistics (D.Z.), North Carolina State University, Raleigh, North Carolina 27695