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Menstruation is the periodic shedding(usually monthly)of the lining of the womb.Generally speaking,menstruation starts during puberty around 10-16 years old and continues until the menopause about 45-55 years old.An average menstrual cycle is about 28 days,but it may vary between 24-35 days.According to a study most women have more than 500 periods in a lifetime.Any abnormality of the above menstrual conditins can be regarded as irregular.