In ischemic heart disease, blood vessels that supply the heart muscle become blocked. If blood flow to an area of heart muscle is blocked for a prolonged period, that area of heart muscle dies and is replaced by scar tissue, a condition commonly known as a heart attack. Blockages in blood flow that do not last long enough to cause the death of heart muscle can cause a type of chest pain known as angina. Before menopause, women are much less likely than men of similar age to have ischemic heart disease. People believe that in premenopausal women, high levels of ovarian hormones such as estrogen reduce the risk for ischemic heart disease. After menopause, these hormone levels decrease, and ischemic heart disease in women becomes more common. In trying to understand how hormones protect premenopausal women from heart disease, scientists have wondered if fluctuations in hormone levels during the menstrual cycle might play some role in heart disease.