The Physics Factbook™
Edited by Glenn Elert -- Written by his students
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|Bibliographic Entry||Result |
|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 from20 to 35 days, the average cycle lasts for 28 days.During this time, the female reproductive system carries out theprocesses of the menstrual cycle, which occurs in phases. Themenstrual cycle begins in females at puberty, which takes placebetween the ages of 10 and 16. It will continue until a womanreaches menopause, at which time the ovaries stop releasing matureeggs. This occurs between the ages of 40 and 60.
A baby girl is born with 40,000 to 300,000 immature eggs inher ovaries, and beginning at puberty, one mature egg will bereleased about every 28 days. This is known as ovulation,and it does not occur until the second phase of the menstrualcycle. The development of an egg is stimulated by the follicle-stimulatinghormone (FSH). This hormone causes the follicles in the ovaryto develop, which in turn causes the secretion of the female hormoneknown as estrogen. The thickening of a lining with mucus and bloodvessels on the walls of the uterus is stimulated by estrogen.This is the proliferative phase of the cycle and it lasts about10 to 14 days. The uterine lining is called the endometriumand it prepares the uterus for a fertilized egg. The developmentof the follicles and ovulation are part of the ovarian cycle,which is not part of the menstrual cycle but instead occurs parallelto it. As the uterine lining continues to thicken, the level ofestrogen in the blood increases, and this causes the secretionof FSH to decrease while inducing the secretion of luteinizinghormone (LH). This hormone stimulates ovulation. When the eggis released, the follicles are broken, and the corpus luteum isformed, which secretes progesterone. It causes the continued growthof the endometrium. This is all included in the secretory phase,which also lasts about two weeks. If as the mature egg travelsthrough the Fallopian tubes to the uterus, it is not fertilizedby sperm, the secretion of LH decreases, and the corpus luteumno longer secretes progesterone. The uterine lining breaks down,and it is released with the egg and a little blood during thephase known as menstrual flow, or menstruation. This lasts for3 to 7 days, and the menstrual cycle begins again.
The cycle of the moon's phases, or synodic cycle, lasts 29.5 daysfrom new moon to new moon.The period of the sidereal cycle ofthe moon, which is the time it takes for the moon to completean orbit of the earth, is 27.3 days. The average of the periodof the synodic and sidereal cycle is 28 days, which is theaverage period of the human menstrual cycle. This correlationhas prompted some to believe that the moon "controls"the female reproductive system. In the culture of the ancientRomans, the goddess of the moon, Diana, cared for women, especiallythose who were pregnant.
In most cases, the period of the menstrual cycle is predictableafter it stabilizes during the first few years of puberty. Thisconsistency is important to women because an irregularity in thecycle 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 familiarwith the period of their menstrual cycles in order to increasethe chances of fertilization. Usually, a woman is most fertilebetween days 10 and 17 of her cycle, because during thattime, the egg is in the Fallopian tubes.
Tiffany Eng -- 2001
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