Routinely collected pre- and postnatal clinical data will be entered into a digital database and echocardiographic examinations will be uploaded to a server. A core lab will measure and analyze all echocardiographic examinations according to protocol. The growth of the left heart structures and the postnatal outcome will be compared between the intervention and non-intervention groups.~The decision whether a fetal balloon dilatation shall be attempted is not part of the study protocol. The number of examinations of mother/fetus/infant in this study is not different from the number of examinations that will be recommended for someone choosing not to be part of this study. Participation in the study does not affect the care and treatment mothers and fetuses are receiving during pregnancy, nor how the infant is examined and treated after birth.
In one of the most severe congenital heart defects, hypoplastic left heart syndrome (HLHS), the left ventricle is underdeveloped and the prognosis is worse than in most other heart defects. The underdevelopment can occur gradually during fetal growth caused by a narrowing of the aortic valve. At some international centers, such fetuses are treated with a balloon dilation of the narrowed valve, but there is no scientifically sound evidence that this treatment is effective.~The aim of this study is: 1/ to evaluate whether balloon dilation during the fetal period of a narrowed aortic valve can reduce the risk of the left ventricle becoming underdeveloped and the baby being born with a so-called univentricular heart (HLHS); 2/ to investigate whether such treatment improves the prognosis for this group of children with a very complex and severe heart defect and 3/ to also describe side effects and risks in fetuses and mothers of the fetal procedure.