A period is what happens at the end of a girl's menstrual cycle, the body's process of preparing for a possible pregnancy. Girls begin having periods approximately monthly once they reach childbearing age, usually between 11 and 14 years.
WebMD explains that each month a girl's uterus grows a new lining to prepare to nourish a fertilized egg. During a cycle that does not result in pregnancy since no egg was fertilized, the uterus sheds its lining. This shedding is the monthly bleeding known as a girl's period.
The menstrual cycle is controlled by hormones, especially estrogen and progesterone. Estrogen builds up the uterine lining. Progesterone affects the shedding of the lining. The hormones involved in the menstrual cycle can cause other symptoms besides bleeding. Some girls experience emotional tension or anger, tender breasts, acne, low levels of energy, water weight gain or abdominal cramps before or during their period. After the first few days of a girl's period, these symptoms often go away.
Girls use tampons or pads to manage the bleeding, which normally lasts up to seven days. To help with other period-related symptoms, girls can use a heating pad, a warm bath or over-the-counter pain medications such as ibuprofen or naproxen.