1. Department of Obstetrics and Gynecology, The Lundquist Institute at Harbor—UCLA Medical Center, Torrance, CA, USA
2. California Preterm Birth Initiative, School of Medicine, University of California, San Francisco, San Francisco, CA, USA
3. Department of Obstetrics, Gynecology & Reproductive Sciences, School of Medicine, University of California, San Francisco, San Francisco, CA, USA
4. Department of Epidemiology and Biostatistics, School of Medicine, University of California, San Francisco, San Francisco, CA, USA
5. Institute for Circumpolar Health Studies, University of Alaska, Anchorage, Anchorage, AK, USA
6. Department of Human Ecology, College of Agricultural and Environmental Sciences, University of California, Davis, Davis, CA, USA