[1] Univ Pittsburgh, Sch Publ Hlth, Dept Epidemiol, 5128 Publ Hlth,130 DeSoto St, Pittsburgh, PA 15261 USA
[2] Univ Pittsburgh, Sch Med, Dept Obstet Gynecol & Reprod Sci, Pittsburgh, PA 15260 USA
[3] Univ Waterloo, Sch Publ Hlth & Hlth Syst, Waterloo, ON, Canada
[4] Emory Univ, Rollins Sch Publ Hlth, Dept Epidemiol, Atlanta, GA 30322 USA