This extremely pleasurable, involuntary response lasts a few seconds. Beginning menstruation is one of the most significant events of puberty for young women and usually begins between the ages of 10 and 16. It's important to understand why and how it occurs. The ovaries usually produce one egg each month. If the egg and sperm join together in the fallopian tube, fertilization occurs.
Pregnancy then begins if the fertilized egg attaches itself to the lining of the uterus.
Most of the time this doesn't happen, however. The egg does not meet a sperm and it begins to break apart before it gets to the uterus. In this case, the tissue and blood which line the uterus aren't needed and they flow out the uterus, through the cervix and vagina and out of the body. One of the most important things for girls to understand is that menstruation is normal and healthy. While blood is a sign of injury or disease in other circumstances, in this case, it's a sign they are growing up.