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 < 0.001), but not boys (1.10; 95 % CI 0.97–1.25; P = 0.14) (Pinteraction = 0.02). Among girls, high fetal growth was associated with disease onset before age 5 years (odds ratio per 1 SD, 1.47; 95 % CI 1.28–1.69; P < 0.001), but not beyond (1.00; 95 % CI 0.76–1.31; P = 0.99). No clear associations were found for gestational age at birth or other perinatal factors. In this large cohort study, high fetal growth was associated with Wilms tumor before age 5 years among girls. These findings suggest that early-life growth factor pathways for Wilms tumor may be more common among girls than boys. Further elucidation of these mechanisms may reveal better targets for prevention or treatment of specific subtypes of Wilms tumor.