1. Department of Ambulatory Care and Prevention, Harvard Pilgrim Health Care, Harvard Medical School, and Harvard Vanguard Medical Associates, Boston, Massachusetts.
2. Department of Pediatrics, University of California, San Francisco, San Francisco, California.
3. Department of Obstetrics, Gynecology and Reproductive Sciences, and Epidemiology and Biostatistics, University of California, San Francisco, San Francisco, California.
4. Perinatal Research Unit, Kaiser Permanente Medical Care Program, Division of Research, Oakland, California.
5. Division of General Medicine and Primary Care, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts.
6. Institute for Health Policy Studies, University of California, San Francisco, San Francisco, California.