1. Department of Obstetrics and Gynecology, Division of Maternal Fetal Medicine, Medstar Washington Hospital Center, Washington, DC, USA;
2. Department of Gynecology and Obstetrics, Division of Maternal-Fetal Medicine, Johns Hopkins School of Medicine, Baltimore, MD, USA;
3. Department of Biostatistics and Bioinformatics, Medstar Health Research Institute, Hyattsville, MD, USA