Menstrual periods may be very irregular at the other end of the menstrual years. Many women realize that they are approaching perimenopause and menopause when their otherwise regular periods become irregular. Menopause occurs when it has been 12 months since you have had a menstrual period. (WebMd)

Most women associate menopause with the lack of menstruation, as well as the symptoms that are most prevalent roughly five years before and five years after their last period. The few years before and after the last period are known as "perimenopause" and "climacteric". Perimenopause is heralded by the onset of irregular periods. Menopause is the last menstrual cycle. But for as many as 10 years prior to menopause (this time is called perimenopause) you may experience a variety of symptoms. Some women experience heavier flows, some lighter flows; some experience hot flashes, others night sweats. (Tampax^®)

Estrogen and progesterone regulate the thickening of the lining of the uterus that's shed each month during menstruation. When levels of these hormones get out of whack, the lining can become thicker than usual. That, of course, results in heavier bleeding when the lining is shed. (Source)