1. Division of Maternal Fetal Medicine, Department of Obstetrics, Gynecology, and Reproductive Sciences, University of California, San Francisco, San Francisco, CA, USA
2. Department of Pediatrics, University of California, San Diego, La Jolla, CA, USA
3. UCSF California Preterm Birth Initiative, University of California, San Francisco, San Francisco, CA, USA
4. Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Department of Epidemiology, The Ohio State University, Columbus, OH, USA
5. Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Columbia University Irving Medical Center, New York, NY, USA
6. Department of Epidemiology and Biostatistics, University of California, San Francisco, San Francisco, CA, USA