Christine Dehlendorf1, Loretta Gavin2, Susan Moskosky2. 1. Departments of Family and Community Medicine, Obstetrics, Gynecology and Reproductive Sciences, and Epidemiology and Biostatistics, University of California, San Francisco, 995 Potrero Avenue, San Francisco, CA 94110, USA. Electronic address: christine.dehlendorf@ucsf.edu. 2. The Office of Population Affairs, U.S. Department of Health and Human Services, Washington, DC, USA.