1 Department of Obstetrics, Gynecology, and Reproductive Sciences, University of California, San Francisco, San Francisco, CA. Electronic address: mary.norton@ucsf.edu.
2 Department of Obstetrics, Gynecology, and Reproductive Sciences, University of California, San Francisco, San Francisco, CA.
3 Department of Epidemiology and Biostatistics, University of California, San Francisco, San Francisco, CA.