As a person ages, their ovaries begin to make hormones less regularly. In some months, there may not be ovulation. Without ovulation, progesterone isn't secreted, so a normal period doesn't occur. The menstrual cycle will be harder to predict. But some months ovulation may occur, and pregnancy can occur. Over time, the ovaries stop working. This can cause symptoms. Some people who have had their uterus taken out (hysterectomy) but still have ovaries may still have symptoms. When estrogen levels reach their lowest point, periods will stop completely. This is menopause.