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 (narrowing of the aortic valve) through the same degenerative process that occurs in normal valves. The main difference is that people who have bicuspid valves will typically develop stenosis in their 30s or 40s and people who have normal valves may develop stenosis after age 50 or 60.