Decrease in hormones:
Estrogen and progesterone are the hormones that are required for menstruation. Your ovaries produce these hormones.
With increasing age, your ovaries start producing less amounts of these hormones and your chances of becoming pregnant decline. At the age of 40 years, your periods may become irregular, and eventually, as you reach 51 years of age, your ovaries stop producing eggs and your periods stop.
Removal of uterus with the ovaries left intact is known as hysterectomy. This does not cause immediate Menopause. Your periods stop, but your ovaries will continue to release eggs and produce hormones. If you have undergone a total hysterectomy & bilateral oophorectomy, where both the uterus and the ovaries are removed, you will experience immediate menopause. Your periods stop immediately.
Cancer therapies can cause menopause.