Menstrual cycle is the series of monthly changes a woman's body goes through in preparation for the possibility of pregnancy. Every month, one of the ovaries releases an egg and this process is called ovulation. Hormonal changes prepare the uterus for pregnancy at the same time. If the ovulation takes place and the egg isn't fertilized, then the lining of the uterus sheds through the vagina. This is a menstrual period. The menstrual cycle, when is counted from the first day of one period to the first day of the next, isn't the same for every woman. Menstrual flow might occur for every 21 to 35 days and last two to seven days. For the first few years when the menstruation begins, long cycles are common. However, menstrual cycles tend to shorten and become more regular as you age is passed.