Estrogen production drops dramatically. Menopause frequently is preceded by a few years of perimenopause. Menstrual cycles are frequently shorter. Menstrual cycles are often irregular. Many women will begin having hot flashes.
Osteoporosis is proven to be associated with estrogen deficiency.
The answer to this question is complicated. Estrogen treatment decreases the risk of dying from breast cancer. However, estrogen treatment increases the risk of developing breast cancer. There is a minimal increase in the risk of blood clots. Estrogen treatment reduces the risk of dying from heart disease, colon cancer, and breast cancer.