The development of a menstrual cycle is another part of puberty for girls. This cycle prepares her body for pregnancy each month; if no pregnancy occurs during that cycle then hormones signal to the body to end the cycle. When the cycle ends, blood and other nutrients that have lined the uterus are released from the vaginal opening. This is called a period.

For each menstrual cycle (which lasts about 28 days but can vary widely among teens), an egg (ovum) matures and is released from one of a girl’s two ovaries (located inside the abdomen, very low in the body.)

The egg travels from the ovary, through a fallopian tube (she has one for each ovary) toward her uterus. As the egg travels, it sends hormonal signals to the body, telling it to develop a thick lining of blood and tissue inside the uterus. The job of this lining is to help the egg attach to her uterus, should it meet up with a sperm and become fertilized. If this happens, the fertilized egg latches onto the thick lining of tissue and blood, and a pregnancy begins.

If there’s no fertilization by sperm, there’s no pregnancy and no need for that thick lining to sustain a pregnancy, so the body gets rid of it. This shedding of the lining, a period, can last anywhere from a few to several days. In all, it’s not a lot of blood that’s released, even though it seems like it sometimes.