In the years leading up to menopause, usually from the ages of 45 to 54, a woman's ovaries start to shrink, and the levels of the female hormones they produce, estrogen and progesterone, become irregular.
This can cause problems for some women, such as hot flashes, sweats and sleep disorders, and doctors often prescribe hormone replacement therapy to alleviate them.
Leaving aside for the moment the question of what, in an evolutionary sense, menopause is for (that is, what is its function?), when it happens the risk of a number of diseases increases. The rate of bone loss speeds up, leading to a greater risk of osteoporosis, and the risk of heart disease also rises.