University of Iowa, Department of Epidemiology, United States. Electronic address: Kelli-ryckman@uiowa.edu. University of Iowa, Department of Epidemiology, United States. University of Iowa, Department of Biostatistics, United States. University of California, San Francisco, Department of Epidemiology & Biostatistics, United States; UCSF California Preterm Birth Initiative, United States. UCSF California Preterm Birth Initiative, United States; University of California San Francisco, Department of Pediatrics, United States. University of Iowa, Department of Pediatrics, United States.