Inclusion Criteria:

• A. All of the following echocardiographic criteria need to be satisfied between 23+0 and 31+6 weeks (z-scores according to Schneider et al):
• 1. Aortic valve stenosis with antegrade flow through the valve
• 2. Predominantly left-to-right shunt at the atrial level
• 3. Predominantly retrograde flow in the aortic arch between the first two brachiocephalic vessels
• 4. Qualitatively depressed left ventricular function
• 5. Left ventricular end-diastolic diameter Z-score \> ±0
• 6. Left ventricular inlet length in diastole :
• 1. Gestational age ≤ 24+6: Z-score \> ±0
• 2. Gestational age 25+0 to 27+6: Z-score \> -0.75
• 3. Gestational age ≥ 28+0: Z-score \> -1.50
• 7. Mitral valve diameter in diastole Z-score \> -2.0
• B. All of the following postnatal treatment options need to be available: 1. Surgical or catheter based aortic valvotomy 2. Ross-Konno surgery 3. Norwood or hybrid stage-one surgery

Exclusion Criteria:

• 1. Any associated cardiac defect except persistent left superior vena cava and coarctation of the aorta
• 2. Any significant (i.e. that might influence outcome) extracardiac anomaly and/or known chromosomal aberration. Also, if such a condition is present at inclusion but diagnosed only after birth the case will be retrospectively excluded.