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Perimenopause means "around the time of menopause."
Postmenopause is the entire period of time that comes after the last menstrual period.
Menopause is the time in a woman's life when the function of the ovaries ceases. The ovaries are the main source of female hormones. The hormones also regulate the menstrual cycle and pregnancy. Estrogens also protect the bone. Therefore, a woman can develop osteoporosis later in life when her ovaries do not produce adequate estrogen.