1. Department of Epidemiology, University of Iowa, Iowa City, IA, USA
2. Department of Pediatrics, University of California San Diego, La Jolla, CA, USA
3. California Preterm Birth Initiative, University of California San Francisco, San Francisco, CA, USA
4. Department of Biostatistics, University of Iowa, Iowa City, IA, USA
5. Department of Pediatrics, University of Iowa, Iowa City, IA, USA
6. Department of Family Health Care Nursing, University of California San Francisco, San Francisco, CA, USA
7. Department of Epidemiology and Biostatistics, University of California San Francisco, San Francisco, CA, USA