[1] Boston Univ, Dept Obstet & Gynecol, Div Maternal Fetal Med, Sch Med,Boston Med Ctr, Boston, MA 02215 USA
[2] Boston Univ, Dept Epidemiol, Sch Publ Hlth, Boston, MA USA
[3] Aetion Inc, Sci Res & Strategy, Boston, MA USA
[4] Boston Univ, Boston Med Ctr, Dept Anesthesiol, Sch Med, Boston, MA USA
[5] Boston Univ, Dept Epidemiol, Sch Publ Hlth, Boston, MA USA
[6] Stanford Univ, Dept Anesthesiol Perioperat & Pain Med, Sch Med, Palo Alto, CA USA
[7] Stanford Univ, Dept Epidemiol & Populat Hlth, Sch Med, Palo Alto, CA USA
[8] Boston Univ, Boston Med Ctr, Dept Pediat,Sch Med, Div Newborn Med, Boston, MA USA