The time from the beginning of one period to the next is known as the menstrual cycle. Approximately once every month, the brain sends a message via the hormones telling one ovary to release an egg. While the egg is getting ready to be released, the lining of the uterus (the womb) begins to thicken. If the egg is not fertilised, this thickened lining is not needed and it leaves the body via the vagina. This is known as menstruation, or having a period.