The menstrual cycle is a periodic succession of phenomena recurring cyclically. These phenomena aim to prepare the mature female genital organ to allow the development of a fertilized egg. The monthly period begins around age 12 on average and only becomes regular around 3 years after onset. The duration of the menstrual cycle is extremely variable from one woman to another, but also from one month to another. The average menstrual cycle lasts 28 days, but a normal menstrual cycle can be between 23 and 35 days. At menopause, the events of the menstrual cycle stop.