作者单位:Univ N Carolina, Div Maternal Fetal Med, 3010 Old Clin Bldg,CB 7516, Chapel Hill, NC 27599 USA[1]Duke Univ, Div Maternal Fetal Med, Durham, NC USA[2]Univ Utah, Div Maternal Fetal Med, Salt Lake City, UT USA[3]Vanderbilt Univ, Div Maternal Fetal Med, 221 Kirkland Hall, Nashville, TN 37235 USA[4]