1 School of Medicine and Health Sciences, George Washington University, Washington, DC.
2 Office of Clinical Research, George Washington University, Washington, DC.
3 Department of Obstetrics and Gynecology, George Washington University, Washington, DC.
4 Department of Anesthesiology and Critical Care Medicine, George Washington University, Washington, DC.
5 Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, George Washington University, 2150 Pennsylvania Ave., NW, 6A Washington, DC 20037. Electronic address: [email protected].