The most significant change during puberty for many girls is their first period. Menstruation is a turning point in your development from a child to a teenager. Among other things, it means you are capable of becoming pregnant. Your first period can be unexpected, and you might be surprised to find that you have some bleeding from your vagina. Many young women might feel frightened by the sight of this bleeding or embarrassed if it causes a stain on their underwear or clothing. It's important to remember that this is a natural process and something that makes being a woman special. You can avoid any embarrassing situations by talking to your mom or another adult (even some dads know about these things, too!) about being prepared for your period.
Your first period is likely to occur between the ages of nine and 16. It usually lasts for three to seven days and then stops until the next period begins-usually about 21 to 28 days after your period started. This timeframe-from the first day you begin to bleed until the first day of your NEXT menstrual period-is called your "menstrual cycle."
During your menstrual cycle, one of your two ovaries releases one microscopic egg, called an ovum. (Your ovaries are reproductive organs approximately one and a half inches long and located in your lower abdomen, one on each side of your uterus; ovaries also release hormones that help to control your menstrual cycle.) The egg's release from the ovary is called "ovulation," and it usually happens in the middle of your cycle-around day 12 to 14 in a 28-day cycle. Ovulation can be irregular, though, when you first start having your period.
The egg then moves through one of the two fallopian tubes (the two tubes attached to the top of the uterus that lead to the ovaries). At the same time, body tissues and blood cells are beginning to line the walls of your uterus, forming a thin layer of material that will eventually be shed as your period. The amazing thing is, you won't feel any of this happening.
If you were to have sexual intercourse at this time, and sperm from a male partner would fertilize your egg on its way to the uterus, you would become pregnant. The egg would attach itself to the lining of the uterus and a fetus would grow inside of you. However, if sperm does not fertilize the egg, your body does not need this lining to support the fertilized egg. So, hormones trigger a different process and this lining gently falls away from the walls of your uterus and is released from your body through your vagina. This is often called the menstrual flow or period.