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