The period, scientifically called menstruation, is the shedding of blood and components of the endometrium each month. In the female the first period usually occurs at age 12 to 13 years, but sometimes it can start from the age of 8 years or be delayed up to 16 years old.
The cycle period is usually 28-30 days from the beginning of one period to the beginning of the next. This period is considered normal even if the cycle lasts from 21 to 35 days. It is also normal for a woman’s cycle to vary from month to the next, probably 2 to 3 days up or down.