Perimenopause describes the time when a woman may begin to experience some of the symptoms that can occur with menopause, such as hot flashes. ("Peri" means "around" or "about.") Her ovaries are slowly shutting down production and periods may become less frequent. Perimenopause usually starts when a woman is in her 40s.
While the term menopause is usually used to describe the entire transitional midlife phase in a woman's life, it does not officially begin until very specific criteria are met: A woman is truly menopausal once she has gone one year without having a menstrual period. Technically speaking, that means a woman is postmenopausal for the rest of her days, which can be up to one third of her life.