1 Department of Epidemiology & Biostatistics, University of California, San Francisco, School of Medicine, and the UCSF California Preterm Birth Initiative, San Francisco, California.
2 Department of Obstetrics, Gynecology & Reproductive Sciences, University of California, San Francisco, School of Medicine, and the UCSF California Preterm Birth Initiative, San Francisco, California.
3 Department of Epidemiology, College of Public Health, University of Iowa, Iowa City, Iowa.