Perinatal Epidemiology Research Unit, Departments of Pediatrics and Obstetrics & Gynaecology, Dalhousie University, Halifax, NS, Canada. Electronic address: stefan.kuhle@dal.ca. Division of Paediatric Epidemiology, Institute of Medical Biostatistics, Epidemiology and Informatics, University Medical Centre of the Johannes Gutenberg University, Mainz, Germany.