[1] Univ Michigan, Sch Publ Hlth, Dept Epidemiol, Ann Arbor, MI 48109 USA
[2] Univ Michigan, Ctr Stat Consultat & Res, Ann Arbor, MI 48109 USA
[3] Wayne State Univ, Sch Med, Div Infect Dis, Detroit, MI USA
[4] Thomas Jefferson Med Coll, Dept Obstet & Gynecol, Philadelphia, PA USA