[1] Univ North Carolina, UNC Blood Res Ctr, Sch Med, Dept Pathol & Lab Med, Chapel Hill, NC 27599 USA
[2] George Washington Univ, Sch Med & Hlth Sci, Dept Obstet & Gynecol, Div Maternal Fetal Med, Washington, DC USA
[3] INOVA Hlth Syst, Dept Obstet & Gynecol, Div Maternal Fetal Med, Falls Church, VA USA