1. Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Sidney Kimmel Medical College at Thomas Jefferson University, Philadelphia, PA, USA;
2. Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Obstetrix Medical Group, Seattle, WA, USA;
3. Division of Biostatistics, Das Statistical Consulting, San Francisco, CA, USA