Menstruation or period, is normal vaginal bleeding that a woman's body goes through in preparation for the possibility of pregnancy. Each month, one of the ovaries releases an egg, a process called ovulation; at the same time, hormonal changes prepare the uterus for pregnancy. If ovulation takes place and the egg is not fertilised, the lining of the uterus sheds through the vagina. This is a menstrual period.

Thus, menstrual blood is partly blood and partly tissue from inside the uterus, which passes out of the body through the vagina. The menstrual cycle, which is counted from the first day of one period to the first day of the next, isn't the same for every menstruator. Menstrual flow might occur every 21 to 35 days and last two to seven days. For the first few years, longer cycles are common. 

Though menstrual cycles tend to shorten and become more regular as a person ages. Hence, a menstrual cycle might be regular; about the same length every month or somewhat irregular, and period might be light or heavy, painful or pain-free, long or short, and still be considered normal. Within a broad range, "normal" is what is normal for each person.