Congenital aortic stenosis refers to a narrowed aortic valve with varying degrees of obstruction. Usually the valve is bicuspid, but other variations exist. Aortic stenosis is a spectrum in which the degree of obstruction ranges from mild to severe. Congenital aortic stenosis occurs due to improper development of the aortic valve in the first 8 weeks of fetal growth. It can be caused by a number of factors, though, most of the time, this heart defect occurs sporadically (by chance), with no apparent reason for its development. The mean gradient across the aortic valve increased by an average of 6.3 mm Hg per year, and the end-systolic diameter of the left ventricle increased by 1.9 mm per year. The rate of increase in gradient was slower in people with more severe stenosis at baseline. Aortic stenosis occurs when abnormalities of the aortic valve lead to narrowing and obstruction between the left ventricle and the aorta. The most common abnormality occurs when the aortic valve has only two (instead of three) leaflets. This is called a bicuspid aortic valve (or BAV). Often the valve leaflets are thickened and less pliable than normal, and the lines of separation between them (or "commissures") are fused together to a variable degree. When the aortic valve  does not open freely, the left ventricle must work harder to eject blood into the aorta.