Some people are born with a bicuspid aortic valve, in which the aortic valve — located between the lower left heart chamber (left ventricle) and the main artery that leads to the body (aorta) — has only two (bicuspid) cusps instead of three. People may also be born with one (unicuspid) or four (quadricuspid) cusps, but these are rare.

A bicuspid aortic valve may cause the heart's aortic valve to narrow (aortic valve stenosis). This narrowing prevents the valve from opening fully, which reduces or blocks blood flow from the heart to the body. In some cases, the aortic valve doesn't close tightly, causing blood to leak backward into the left ventricle (aortic valve regurgitation). Most people with a bicuspid aortic valve aren't affected by valve problems until they're adults, and some may not be affected until they're older adults. Some children with bicuspid aortic valves may have valve problems.

Some people with a bicuspid aortic valve may have an enlarged aorta — the main blood vessel leading from the heart. There is also an increased risk of aortic dissection.