Menstruation is the natural part of the reproductive cycle in which blood from the uterus exits through the vagina. It is a completely normal process and not an illness. Menstruation is also called a girls/woman’s “period”. It usually lasts between three and seven days. Menstruation happens for most women about once a month. It is a sign that a girl can now become pregnant. Just as some girls begin puberty earlier or later than others, the same applies to periods. Some girls may begin to menstruate as early as age nine or 10, and some may not get their first period until a few years later.