Any woman with mitral stenosis should be followed by a perinatologist in close association with her cardiologist. This is a very severe complication of pregnancy. Mitral regurgitation, or leaking back of blood into the left atrium, is usually well tolerated in pregnancy. It's not usually due to rheumatic heart disease, but has many causes. There are usually no fetal effects from this valvular insufficiency.
Rheumatic disease is the culprit.
Of these two, aortic insufficiency is the more common. It is usually well tolerated in pregnancy, and if so, is not harmful to the unborn baby. Aortic stenosis of a mild to moderate degree is usually well tolerated in pregnancy, and severe disease is usually not likely while women are of childbearing age. But if it is severe and a woman is pregnant, the biggest problem seems to be the inability to compensate for exertion, since the obstructive nature of the valve tends to limit the amount of output from the heart to the rest of the body. With this decrease in output, if the amount of blood coming into heart is decreased as well, there could be a big drop-off in blood supply to the mother's brain or the baby's placenta.