The periodic shedding of endometrium, accompanied by bleeding, that occurs in women who are not pregnant. It usually begins at puberty and continues until the menopause. Menstruation occurs at the end of the menstrual cycle, which usually lasts for 28 days (the normal range is 21–35 days). At the beginning of the cycle, a hormone from the pituitary gland stimulates an egg follicle in an ovary to mature. The follicle secretes oestrogen hormones, which make the endometrium thicken. Ovulation (release of an egg from the follicle) usually occurs in the middle of the menstrual cycle. The empty follicle also produces progesterone hormone, which makes the endometrium become swollen and thick with retained fluid. This enables a fertilized egg to implant in the endometrium. If pregnancy fails to occur, the production of oestrogens and progesterone diminishes. The endometrium is then shed about 14 days after ovulation. Uterine contractions force the menstrual discharge to be expelled into the vagina, accompanied by bleeding, which may last for 1–8 days.
Menstruation |
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