Perinatal risk factors including high birth weight have been associated with Wilms tumor in case-control studies. However, these findings have seldom been examined in large cohort studies, and the specific contributions of gestational age at birth and fetal growth remain unknown. We conducted the largest population-based cohort study to date consisting of 3,571,574 persons born in Sweden in 1973-2008, followed up for Wilms tumor incidence through 2009 to examine perinatal risk factors. There were 443 Wilms tumor cases identified in 66.3 million person-years of follow-up. After adjusting for gestational age and other perinatal factors, high fetal growth was associated with increased risk of Wilms tumor among girls (hazard ratio per 1 standard deviation (SD), 1.36; 95% CI 1.20-1.54; P