OBJECTIVE: To assess whether the development of children with varying severity of congenital heart defect (CHD) differs from that of children without CHD at age 6 months. STUDY DESIGN: A total of 236 children with CHD were compared with 61 032 children from the Norwegian Mother and Child Cohort Study conducted by the Norwegian Institute of Public Health. Diagnostic and treatment information was retrieved from a nationwide CHD registry. Four groups of CHD were distinguished: mild (n = 92), moderate (n = 50), severe (n = 70), and CHD with comorbidity (n = 24). At child age 6 months, the children's mothers reported on motor and social development by using the Mother and Child Questionnaire. RESULTS: After adjusting for confounders (ie, birth weight), severe CHD increased the odds of gross motor impairment (odds ratio [OR], 3.78; 95% CI, 1.97-7.25) and fine motor impairment (OR, 2.04; 95% CI, 0.96-4.33). CHD with co-morbidity (eg, intestinal malformations) increased the odds of gross motor impairment (OR, 3.00; 95% CI, 0.95-9.51), fine motor impairment (OR, 5.47; 95% CI, 2.03-14.74), and social impairment (OR, 3.43; 95% CI, 1.40-8.41). CONCLUSION: Increased odds of motor impairment are present already in infancy in severe CHD and CHD with comorbidity. CHD with comorbidity increases the odds of social impairment.