According to present knowledge, ovaries already contain all the egg cells women will have in their lifetime when they are born. These egg cells lie in small pockets called follicles. Once puberty starts, different hormones cause the first follicle to mature and release an egg cell (ovum).
Hormones are substances produced by the body that act as chemical transmitters. They regulate body functions like the menstrual cycle and body temperature. Hormones also trigger ovulation
Around the time when a mature egg cell has left the ovary, a woman can get pregnant. The egg cell travels down the fallopian tube into the uterus. The moment the egg cell leaves the ovary is called ovulation. Once a girl has had her first monthly period, ovulation usually occurs once a month
During the monthly hormone cycle the mucous membranes lining the inside of the uterus prepare for the possibility that an egg might be fertilized and settle in the uterus. The mucous membranes supply nutrients to the embryo if the woman gets pregnant. If the egg cell is not fertilized, it dies.
At the end of the cycle some blood vessels in the mucous membranes of the uterus open up for some time, and the uppermost mucous layer detaches. In order to discharge it, the muscles in the uterus contract and then relax again in an irregular rhythm. That allows the tissue to detach from the wall of the uterus and be shed through the vagina together with some blood. This is the monthly period, also called menstruation. As long as a woman is not pregnant and is not using hormonal contraceptives, the period usually is a sign that one menstrual cycle has finished and the next one has started. In most women, the period lasts 3 to 5 days.
Even though menstrual fluid may look like a lot when it is on a pad or a tampon, normally only about 20 to 60 ml of blood are shed during the monthly period: that is only about 4 to 12 teaspoons.
Most women begin to have irregular periods when they reach menopause, and they completely stop at an average age of 51. A deciding factor for when menopause will begin is the number of follicles remaining in the ovaries. Until around the age of 40, the number of follicles drops slowly. After that, the number drops quite quickly, until no more follicles mature.