When girls reach puberty they start to bleed a little from the vagina once a month, this is called the period, time of the month, or menstruating. Most girls have their first period when they are between eleven and sixteen, and most women stop menstruating when they are between 45 and 55. The mucous membrane in the womb grows every month, so that it can receive an egg cell if it is fertilised. If there is no fertilisation, the egg cell is pushed out along with cells from the uterine wall and some blood. This bleeding usually lasts from three to eight days, and there are usually 21 to 35 days between periods.