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The Menstrual Cycle

For over half of our lives, most women experience the recurring changes of the menstrual cycle. As young girls our ovaries do not yet perform their adult reproductive role. At some point, usually between 9 and 14 years of age, the brain begins sending messages to the ovaries that it is time to begin cyclical production of the two main female hormones, estrogen and progesterone. Actually, men have small amounts of female hormones too. Women also have some testosterone.
Estrogen and progesterone, under direction from the brain, work together to orchestrate the maturing and releasing of eggs (ovulation), as well as to prepare the uterus for implantation of a fertilized egg. Should this happen, they continue to work together to allow pregnancy and the development of the embryo. If no fertilized egg is implanted uterus sheds its lining as menstrual blood, and the cycle begins again.
Cycle lengths vary from woman to woman, and will even vary at different times for the same woman. The average length is 28 days. Interestingly, this is the time for one full lunar cycle. Ovulation can be induced by night light, and so women who sleep under natural moonlight often will ovulate with the full moon and menstruate with the new moon. One reason many women experience irregular cycles may be related to artificial light.