作者单位:Univ Calif San Francisco, Dept Epidemiol & Biostat, San Francisco, CA 94143 USA[1]Univ Calif San Francisco, Calif Preterm Birth Initiat, San Francisco, CA 94143 USA[2]Univ Calif San Francisco, Dept Pediat, San Francisco, CA USA[3]Univ Iowa, Coll Publ Hlth, Dept Epidemiol, Iowa City, IA USA[4]