OBJECTIVE To study the risk for congenital anomalies in the first child of women after bariatric surgery compared with all other women giving birth to their first child and divided by maternal body mass index (BMI) groups. METHODS Prospective, population-based register study. METHODS Sweden. METHODS All firstborn children to women born 1973-83 were studied to determine if they had a congenital anomaly and a mother who had undergone bariatric surgery before pregnancy. METHODS A total of 270,805 firstborns; of which 341 had mothers who had had bariatric surgery before delivery. We retrieved information on the women's marital or cohabitation status, smoking, BMI, diabetes and hypertension during pregnancy. METHODS Congenital malformations. RESULTS Of the firstborn children to mothers who had had bariatric surgery before pregnancy, 4.1% (95% confidence interval [95% CI] 2.2-6.0) were malformed compared with 3.4% (95% CI 3.3-3.5) of those whose mothers had not undergone bariatric surgery. The risk for congenital malformation in firstborn children increased with increasing maternal BMI. The adjusted odds ratio (OR) for congenital malformation among children whose mothers' BMI ranged between 25 and 29 kg/m(2) was 1.09 (95% CI 1.03-1.15), whose mothers' BMI ranged between 30 and 34 kg/m(2) was 1.14 (1.05-1.24) and whose mothers' BMI was ≥35 kg/m(2) was 1.30 (95% CI 1.16-1.45) compared with those whose mothers had a normal BMI. Bariatric surgery before pregnancy did not have any effect on the odds ratio for having congenital malformation (OR = 1.09, 95% CI 0.63-1.91). CONCLUSIONS Preconception bariatric surgery does not seem to affect the risk for congenital malformations but a high to very high BMI does appear to increase the risk.