[1] Johns Hopkins Univ, Sch Med, Dept Obstet & Gynecol, Baltimore, MD 21205 USA
[2] Johns Hopkins Univ, Bloomberg Sch Publ Hlth, Dept Epidemiol, Baltimore, MD 21205 USA
[3] Johns Hopkins Univ, Bloomberg Sch Publ Hlth, Dept Environm Hlth Sci, Baltimore, MD 21205 USA
[4] Columbia Mailman Sch Publ Hlth, Childrens Ctr Environm Hlth, New York, NY USA
[5] Ctr Dis Control & Prevent, Div Sci Lab, Natl Ctr Environm Hlth, Atlanta, GA USA