A congenital heart defect is a malformation that has been present since birth. The most common heart defect that causes aortic valve stenosis is a bicuspid aortic valve. A normal (tricuspid) aortic valve has three flaps, or leaflets; a bicuspid valve has only two leaflets.

This abnormal valve structure causes rough, turbulent blood flow, which over the years can lead to stenosis through the same degenerative process that occurs in normal valves. The main difference is that someone who has a bicuspid valve will typically develop stenosis in their 30s or 40s, as opposed to people who have normal valves, who develop stenosis after age 50 or 60.

People who have a bicuspid valve are also more likely than other people to get an infection (infective endocarditis) that can cause the aortic valve to become leaky (aortic regurgitation) as well as narrow.

Two other, less common congenital defects can cause aortic valve stenosis:

• A misshapen tricuspid aortic valve. Although the valve has the correct number of leaflets, the leaflets may not all be the same size.
• A unicuspid valve. This valve, which has only one leaflet, can cause severe stenosis during infancy and may be fatal at an early age. A unicuspid valve is the most common cause of death from aortic valve stenosis in children younger than 1 year.