1 Division of Epidemiology, Statistics and Prevention Research Eunice Kennedy Shriver National Institute of Child Health and Human Development National Institutes of Health, Rockville, MD 20852, U.S.A. kims2@mail.nih.gov.
2 Division of Epidemiology, Statistics and Prevention Research Eunice Kennedy Shriver National Institute of Child Health and Human Development National Institutes of Health, Rockville, MD 20852, U.S.A. sundaramr2@mail.nih.gov.
3 Division of Epidemiology, Statistics and Prevention Research Eunice Kennedy Shriver National Institute of Child Health and Human Development National Institutes of Health, Rockville, MD 20852, U.S.A. louisg@mail.nih.gov.
4 National Perinatal Epidemiology Unit, University of Oxford, Oxford 327270, U.K. cecilia.pyper@npeu.ox.ac.uk.