1. Division of Research, Kaiser Permanente Northern California, Oakland
2. Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, The Ohio State University College of Medicine, Columbus
3. Department of Obstetrics and Gynecology, Kaiser Permanente Northern California, Oakland
4. Regional Perinatal Service Center, Kaiser Permanente Northern California, Santa Clara
5. Department of Epidemiology and Biostatistics, University of California, San Francisco