[1] Penn State Univ, Sch Nursing, Hershey, PA 17033 USA
[2] Penn State Univ, Dept Publ Hlth Sci, Coll Med, Hershey, PA 17033 USA
[3] Univ N Carolina, Dept Obstet & Gynecol, Chapel Hill, NC USA
[4] Johns Hopkins Univ, Dept Med, Baltimore, MD USA
[5] Johns Hopkins Univ, Dept Biostat, Baltimore, MD 21205 USA
[6] Johns Hopkins Univ, Dept Epidemiol, Baltimore, MD USA