Friends may also provide information about sexual issues. Unfortunately, much of this information may not be accurate, because many myths are passed from one person to another. Most females begin to menstruate between the ages of 9 and 16. As a girl starts having menstrual cycles, the length of the cycle may vary from month to month as her body adjust to changing hormones. It may take several years to develop a regular cycle.
Menstruation begins when the eggs in a female s ovaries begin to mature.
As an egg matures, the female s body begins building the lining of the uterus. Once the egg is mature, it leaves the ovary and enters the fallopian tube. This process is called ovulation. When an egg is not fertilized, it disintegrates after a few days. Hormone levels drop and cause the lining of the uterus to pass from the body.