Arendt LH, Pedersen LH, Pedersen L, et al. - The study was carried out to investigate the occurrence of major congenital abnormalities in children of women with type 1 and type 2 diabetes, as well as the relationship between glycated haemoglobin (HbA1c) and major congenital malformations in type 1 and type 2 diabetes separately. All singletons born alive from January 1, 2000, to December 31, 2015, in the North Denmark and Central Denmark regions of Denmark and their mothers were included in this register-based study. Data from Danish health registers and the LABKA database were used. Among 314,245 infants studied, 2020 (0.64%) had mothers with type 1 diabetes and 498 (0.16%) had mothers with type 2 diabetes. For both type 1 and type 2 diabetes, the prevalence of major congenital malformations increased progressively with poorer glycemic control during pregnancy, with no obvious safe threshold level.