Menstruation occurs between the ages of 10 and 16, sometimes earlier or later. The menstrual cycle is divided into two consecutive waves of hormones: oestrogen and progesterone. Oestrogen causes the endometrium to thicken in preparation for a possible fertilized egg. If there is no implantation, the endometrium decomposes, forming what are known as menstrual periods. On average, menstruation lasts from 3 to 6 days. This cycle continues on a monthly basis until the menopause.
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