As the rest of you is growing, so are your reproductive organs. Some of these organs are inside your body, particularly in the case of the female. Remember back when we were talking about the pituitary gland in girls and how it makes a hormone called FSH that travels down to the inside of the ovary and stimulates the follicle to make estrogen? Well, as the follicles and their ovum (the eggs inside the follicle) are developing and making more and more estrogen, they are also travelling to the surface of the ovary. When they reach the surface of the ovary they stay just on the inside of it like little blisters. Then, when there is enough estrogen in a girl's body, her pituitary gland slows down its production of FSH and starts making another hormone called LH (luteinizing hormone). This hormone travels through the bloodstream to the ovary and causes one of the tiny ripe ovum to break through its blister on the surface of the ovary.
This is called ovulation. After the ovum has broken loose of the ovary, it travels down the fallopian tube to the uterus. During this whole process, the uterus, which is below the ovaries, has also been growing and changing. At the time that the ovaries have been making estrogen, the walls or lining of the uterus (called the endometrium) have been growing thick with spongy tissue and new blood vessels.
When the ovum breaks off of the ovary, another hormone is produced called progesterone. It runs ahead of the ovum and stimulates the endometrium to grow even thicker and richer with blood and nourishing substances. By the time the ovum reaches the uterus, the endometrium is like a welcoming committee, ready to receive and nourish a fertilized egg and to help it grow and develop into a baby.
When the ovum or egg gets down to the uterus, and if it hasn't been fertilized by a male's sperm, the welcoming committee goes home. In technical terms, that means that the hormone progesterone stops producing, which in turn causes the endometrium to break down.
The blood and spongy tissue falls off the walls of the uterus and collects at the bottom where it trickles down into the vagina and out the vaginal opening. This is called a menstrual period, or menstruating. It usually takes a couple of days or so for the uterus to clean itself out.
For a lot of girls, this first "period" can be very frightening, especially if they aren't expecting it or don't realize what it is. Years ago many girls believed they were hurt internally, and that was why they were "bleeding".
HERE'S A DIAGRAM OF THE FEMALE REPRODUCTIVE SYSTEM.
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