1. Department of Population, Family and Reproductive Health Johns Hopkins Bloomberg School of Public Health Baltimore MD USA
2. Department of Environmental Health Sciences Columbia University Mailman School of Public Health New York NY USA
3. School of Public Health University of California, Berkeley Berkeley CA USA
4. Department of Obstetrics, Gynecology and Reproductive Sciences University of California, San Francisco San Francisco CA USA
5. California Preterm Birth Initiative University of California, San Francisco San Francisco CA USA
6. Department of Epidemiology and Biostatistics Michigan State University East Lansing MI USA
7. Program in Public Health University of California, Irvine Irvine CA USA