[1] Univ Baltimore, Dept Gynecol & Obstet, Baltimore, MD 21201 USA
[2] Univ British Columbia, Dept Obstet & Gynecol, Vancouver, BC V5Z 1M9, Canada
[3] Johns Hopkins Bloomberg Sch Publ Hlth, Dept Biostat, Baltimore, MD USA
[4] Johns Hopkins Bloomberg Sch Publ Hlth, Dept Epidemiol, Baltimore, MD USA
[5] St Johns Mercy Med Ctr, Baltimore, MD USA