Periods usually start about 18 months after your breasts first begin to develop. Hormones produced by the ovaries make the lining of the womb get thicker, to prepare for a fertilised egg to settle in and develop into an embryo. Every 28 days or so (usually) an egg is released from an ovary, and if it is not fertilised, the thickened lining of the womb is shed. There is some bleeding when this happens, and the lining cells and the blood come out through the vagina. This is the period.
A period usually lasts from three to eight days, and often the muscle layer of the womb contracts during the bleeding. If the contractions are strong, they cause the crampy pain many women feel during a period. The pain is usually at its worst in the first couple of days.
About two weeks after the period begins, another egg is released, and the cycle begins again. This is the menstrual ('monthly' in Latin) cycle, which usually takes about 28 days from the start of one period to the start of the next.
There is a lot of variability between women in the length of the period, the length of the cycle, the amount of blood loss and the intensity of the cramps. The hormone changes in the blood just before the period can affect the mood of many women - feeling low, irritable and emotional - until the period begins. This is feeling 'premenstrual'.
For a few women feeling premenstrual, the degree of blood loss and cramps can be very disruptive to everyday life. There is no need for anyone to suffer this much - GPs can offer a range of treatments to help particularly difficult periods.