Having a period is an important part of being a woman — it signifies that you're fertile or capable of creating and having a baby. Most girls start menstruating between the ages of 10 and 13.
If you’re worried about starting your period early or late, talk to your parents or doctor. Everyone is different and what is “normal” for you may be different from what is normal for someone else.
An average cycle lasts 28 days, but it can change from month to month. Your cycle is controlled by hormones that prepare your body for a possible pregnancy every month.
The first day of your cycle is the day you start bleeding. Your body is shedding the lining built up in your uterus. This usually lasts two to seven days. The second day is sometimes the day that women bleed most heavily. While you lose only about 1/4 cup of blood during your period, sometimes it can seem like more.
When the bleeding stops, your ovaries start preparing a new egg to be released, and your uterus starts building up another lining to support a pregnancy (in case the egg is fertilized by the male sperm).