Quanhe Yang is with the Division of Birth Defects and Developmental Disabilities, National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention, Atlanta, Ga. Sander Greenland is with the Departments of Epidemiology and Statistics, University of California, Los Angeles. W. Dana Flanders is with the Department of Epidemiology, School of Public Health, Emory University, Atlanta, Ga.