The good news is, they don't. The bad news is, they're here for a while. On average, a menstrual cycle lasts around 28 to 30 days, give or take a few days (everyone's unique!). There are two reasons that periods stop - pregnancy and menopause. During pregnancy, periods will stop until sometime after the birth of the baby. Menopause is a time in life when older women (40 to 55 years) stop having periods and can no longer become pregnant.