Perimenopause is the time leading up to menopause when you start to notice menopause-related changes–plus the year after menopause. Perimenopause is what some people call “being in menopause” or “going through menopause.” But menopause itself is only one day–the day you haven’t had a period for 12 months in a row. During perimenopause, your ovaries start to shut down, making less of certain hormones (estrogen and progesterone), and you begin to lose the ability to become pregnant. This change is a natural part of aging that signals the ending of your reproductive years.
Source: U.S. Department of Health and Human Services.