This study investigates the association between maternal diabetes and overweight or obesity with congenital heart defects (CHDs) in offspring. Conducted in Finland, the study analyzed data from 620,751 children born between 2006 and 2016 and their mothers. Maternal prepregnancy body mass index (BMI) and diabetes status were assessed, and the odds ratios (ORs) for isolated CHDs were calculated. Key findings include: - Maternal type 1 diabetes (T1D) was associated with a 3.77-fold increased risk of any CHD and was linked to increased risk in 6 out of 9 CHD subgroups. - Maternal overweight was associated with left ventricular outflow tract obstruction (LVOTO) and ventricular septal defects (VSD), while obesity was associated with complex defects and right outflow tract obstruction (RVOTO). - Maternal T1D was the strongest risk factor, with a 3.77-fold increased risk of any CHD compared to no maternal diabetes. - Maternal overweight and obesity had smaller increases in CHD risk but were associated with specific CHD subgroups. - The study suggests distinct teratogenic mechanisms for maternal T1D and overweight/obesity, with T1D affecting cardiac septation and overweight/obesity affecting outflow tract obstruction and complex defects. The study highlights the importance of managing maternal diabetes and addressing overweight/obesity to reduce the risk of CHDs in offspring.This study investigates the association between maternal diabetes and overweight or obesity with congenital heart defects (CHDs) in offspring. Conducted in Finland, the study analyzed data from 620,751 children born between 2006 and 2016 and their mothers. Maternal prepregnancy body mass index (BMI) and diabetes status were assessed, and the odds ratios (ORs) for isolated CHDs were calculated. Key findings include: - Maternal type 1 diabetes (T1D) was associated with a 3.77-fold increased risk of any CHD and was linked to increased risk in 6 out of 9 CHD subgroups. - Maternal overweight was associated with left ventricular outflow tract obstruction (LVOTO) and ventricular septal defects (VSD), while obesity was associated with complex defects and right outflow tract obstruction (RVOTO). - Maternal T1D was the strongest risk factor, with a 3.77-fold increased risk of any CHD compared to no maternal diabetes. - Maternal overweight and obesity had smaller increases in CHD risk but were associated with specific CHD subgroups. - The study suggests distinct teratogenic mechanisms for maternal T1D and overweight/obesity, with T1D affecting cardiac septation and overweight/obesity affecting outflow tract obstruction and complex defects. The study highlights the importance of managing maternal diabetes and addressing overweight/obesity to reduce the risk of CHDs in offspring.