Why does blood when girls have their period?

Menstruation, commonly known as a period, is the regular discharge of blood and tissue from the lining of the uterus. It occurs when the fertilized egg doesn't implant in the uterine wall. In preparation for possible pregnancy, the uterus builds up its lining with blood and tissue. When pregnancy doesn't occur, the built-up lining sheds and exits the body through the vagina. This process usually lasts for several days and repeats once every 21 to 35 days, except during pregnancy and breastfeeding. Menstruation is a typical part of life for individuals with a fertile female reproductive system, although irregular menstruation is common for several months prior to the onset of menopause.