1 Department of Obstetrics and Gynecology, College of Physicians and Surgeons, Department of Epidemiology, Joseph L. Mailman School of Public Health, Columbia University, New York, NY. Electronic address: cande.ananth@columbia.edu.
2 Epidemiology Branch, Division of Intramural Population Health Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD.