1. Department of Combined Internal Medicine and Pediatrics, Los Angeles County+University of Southern California Medical Center, Los Angeles, California, USA.
2. Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA.
3. Department of Biostatistics and Bioinformatics, Duke University Medical Center, Durham, North Carolina, USA.
4. Department of Obstetrics and Gynecology, Duke University School of Medicine, Durham, North Carolina, USA.