1. Division of Maternal-Fetal Medicine, Department of Obstetrics, Gynecology & Reproductive Sciences, University of California, San Diego, San Diego, California, USA.
2. College of Arts and Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA.
3. Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA.
4. School of Nursing, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA.