作者单位:Univ Calif San Francisco, San Francisco, CA 94143 USA[1]Univ Calif Los Angeles, Los Angeles, CA USA[2]Perinatal Qual Fdn, Oklahoma City, OK USA[3]Univ Penn, Philadelphia, PA 19104 USA[4]Columbia Univ, Med Ctr, New York, NY USA[5]Massachusetts Gen Hosp, Dept Obstet & Gynecol, Brookline, MA USA[6]Univ Rochester, Rochester, NY USA[7]