1. School of Public Health, Oregon Health & Science University—Portland State University , Portland, Oregon, USA
2. Department of Obstetrics & Gynecology, Oregon Health & Science University , Portland, Oregon, USA
3. Department of Epidemiology and Population Health, Stanford University School of Medicine , Stanford, California, USA
4. Department of Obstetrics & Gynecology, Vanderbilt University Medical Center , Nashville, Tennessee, USA
5. Department of Pediatrics, Stanford University School of Medicine , Stanford, California, USA
6. Department of Obstetrics & Gynecology, Stanford University School of Medicine , Stanford, California, USA