In the decade between the ages of eight and 18, a girl's body changes into that of a woman. Girls often go through puberty before boys do.
On the inside of your body, the changes are no less dramatic. At some stage, usually around the ages of twelve or thirteen, you will start having your periods. This is called menstruation and this is caused by your hormones.
Your ovaries release an egg, which travels down the fallopian tubes to the uterus. The uterus grows a thick lining of blood, preparing for a possible pregnancy. If you have sex with a man during this time, the egg could be fertilised and might attach itself to the lining of the uterus and the pregnancy will begin.