 Study design: Randomized educational intervention trial. Study population: Children born in Finland and under 6 months of age in 1.7.2017-31.12.2019 with, a major CHD and potentially included in the criteria of endocarditis prophylaxis, or b with a major CHD combined with a chromosomal syndrome, will be offered to participate in the study. Patients will be recruited until 200 patients are obtained at the Helsinki University Childrens Hospital. A control group consisting of approximately 100 healthy children will also be recruited at birth. 