The Physics
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Period of the Human Menstrual Cycle

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Campbell, Neil A., Jane B. Reese, & Lawrence G. Mitchell. Biology. 1987. Ed. Erin Mulligan, 5th ed. New York: Addison Wesley Longman. 1999. "The human menstrual cycle averages 28 days; the estrous cycle of the rat is only 5 days.Bears and dogs have one cycle per year, but elephants cycle several times per year." 28 days
"Menstrual Cycle." Mosby's Medical Encyclopedia. Ed. Jean Williams Brusher & Janet Joers. CD-ROM. Vers. 2.0. New York: TLC Properties, 1997. "The average length of the cycle from the first day of bleeding of one cycle to the first of another is 28 days." 28 days
Wilcox, Allen J., David Dunson, & Donna Day Baird. "The Timing of the Fertile Window in the Menstrual Cycle: Day Specific Estimates From A Prospective Study." British Medical Journal. 321.7271 (2000). "It follows that in the usual menstrual cycle lasting 28 days, the fertile days would fall between days 10 and 17." 28 days
Bruun, Ruth Dowling, & Bertel Bruun. The Human Body: Your Body and How It Works. New York: Random House, 1982. "The entire menstrual cycle, from the ripening of an egg to its discharge, takes 28 days on the average, but wide variations in this schedule are quite common and quite normal." 28 days
What is Menstruation? PeriodWatch. "The average menstrual cycle is 28 days long and, for many women, is very predictable." 28 days

Although the period of the human menstrual cycle can vary from 20 to 35 days, the average cycle lasts for 28 days. During this time, the female reproductive system carries out the processes of the menstrual cycle, which occurs in phases. The menstrual cycle begins in females at puberty, which takes place between the ages of 10 and 16. It will continue until a woman reaches menopause, at which time the ovaries stop releasing mature eggs. This occurs between the ages of 40 and 60.

A baby girl is born with 40,000 to 300,000 immature eggs in her ovaries, and beginning at puberty, one mature egg will be released about every 28 days. This is known as ovulation, and it does not occur until the second phase of the menstrual cycle. The development of an egg is stimulated by the follicle-stimulating hormone (FSH). This hormone causes the follicles in the ovary to develop, which in turn causes the secretion of the female hormone known as estrogen. The thickening of a lining with mucus and blood vessels on the walls of the uterus is stimulated by estrogen. This is the proliferative phase of the cycle and it lasts about 10 to 14 days. The uterine lining is called the endometrium and it prepares the uterus for a fertilized egg. The development of the follicles and ovulation are part of the ovarian cycle, which is not part of the menstrual cycle but instead occurs parallel to it. As the uterine lining continues to thicken, the level of estrogen in the blood increases, and this causes the secretion of FSH to decrease while inducing the secretion of luteinizing hormone (LH). This hormone stimulates ovulation. When the egg is released, the follicles are broken, and the corpus luteum is formed, which secretes progesterone. It causes the continued growth of the endometrium. This is all included in the secretory phase, which also lasts about two weeks. If as the mature egg travels through the Fallopian tubes to the uterus, it is not fertilized by sperm, the secretion of LH decreases, and the corpus luteum no longer secretes progesterone. The uterine lining breaks down, and it is released with the egg and a little blood during the phase known as menstrual flow, or menstruation. This lasts for 3 to 7 days, and the menstrual cycle begins again.

The cycle of the moon's phases, or synodic cycle, lasts 29.5 days from new moon to new moon.The period of the sidereal cycle of the moon, which is the time it takes for the moon to complete an orbit of the earth, is 27.3 days. The average of the period of the synodic and sidereal cycle is 28 days, which is the average period of the human menstrual cycle. This correlation has prompted some to believe that the moon "controls" the female reproductive system. In the culture of the ancient Romans, the goddess of the moon, Diana, cared for women, especially those who were pregnant.

In most cases, the period of the menstrual cycle is predictable after it stabilizes during the first few years of puberty. This consistency is important to women because an irregularity in the cycle can signify important changes in a women's body. For instance, if menstruation does not take place according to the usual cycle, it could mean that the woman is pregnant. Also, women become familiar with the period of their menstrual cycles in order to increase the chances of fertilization. Usually, a woman is most fertile between days 10 and 17 of her cycle, because during that time, the egg is in the Fallopian tubes.

Tiffany Eng -- 2001