Menstruation, also called having a period, occurs as girls get older and their bodies go through changes. They experience their periods when the lining of the uterus sheds. The uterus is the part inside a woman's body where a baby grows during pregnancy. The inner lining of the uterus supports and nourishes the developing baby. If there is no baby, the lining is shed during the woman's period. During this time, blood and other tissue from the uterus leave the body through the vagina. A period can last from a few days to about a week. Doctors often refer to the menstrual cycle, which is the time from one period to another, typically around 28 days. The first day of the menstrual cycle is the first day of a period. As soon as the period ends, the uterus starts to shed its lining, and it begins preparing to receive a fertilized egg. Women's ovaries release tiny eggs, approximately one-tenth the size of a poppy seed. Each woman has two ovaries, one on each side. The fallopian tubes connect each ovary to the uterus. In the middle of the menstrual cycle, an ovary releases an egg. After ovulation, the egg travels down the fallopian tube. This journey takes a few days. Meanwhile, the lining of the uterus gradually thickens to support the development of a fertilized egg. If the egg becomes fertilized, it will implant in the uterus, and the woman will become pregnant. However, if the egg remains unfertilized, the woman is not pregnant, and the egg will start to dissolve. Since the woman isn't pregnant, the lining of the uterus is not needed, and she will have her monthly period. Then the cycle begins again.