作者单位:Univ Michigan, Dept Family Med, Ann Arbor, MI 48104 USA[1]Univ Michigan, Ctr Stat Consultat & Res, Ann Arbor, MI 48104 USA[2]Univ Minnesota, Dept Family & Community Med, Minneapolis, MN USA[3]Univ Michigan, Dept Obstet & Gynecol, Ann Arbor, MI 48104 USA[4]