C V Ananth1,2, E F Schisterman3. 1. Department of Obstetrics and Gynecology, College of Physicians and Surgeons, Columbia University, New York, NY, USA. 2. Department of Epidemiology, Joseph L. Mailman School of Public Health, Columbia University, New York, NY, USA. 3. 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, USA.