1. Division of Reproductive Health, Centers for Disease Control and Prevention, Atlanta, Georgia;
2. Department of Pediatrics, Yale School of Medicine, New Haven, Connecticut;
3. Pregnancy and Perinatology Branch, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland;
4. Data Coordinating Center, Boston University School of Public Health, Boston, Massachusetts; and
5. Department of Pediatrics, Boston University School of Medicine, and Slone Epidemiology Center, Boston University, Boston, Massachusetts