1. Department of Pediatrics, University of California, San Diego, CA, USA;
2. California Preterm Birth Initiative, University of California, San Francisco, CA, USA;
3. Department of Family Health Care Nursing, University of California, San Francisco, CA, USA;
4. Department of Epidemiology & Biostatistics, University of California, San Francisco, CA, USA;
5. Departments of Epidemiology and Pediatrics, University of Iowa, Iowa City, IA, USA;
6. Department of Obstetrics, Gynecology, and Reproductive Sciences, University of California, San Francisco, California, USA