A woman's period starts between 21 and 35 days from the first day of her last period, notes the Office on Women's Health. For teens, the cycle may be 21 to 45 days from the first day of their last period. The average menstrual cycle is 28 days.
A woman's period is important for regulating hormones and plays a vital role in pregnancy. A period occurs when an egg is unfertilized. An unfertilized egg triggers the uterus to shed thickened lining that would have protected a fertilized egg during growth. The Office on Women's Health states that the average period lasts three to five days. Some women have week-long periods.