Discuss the menstrual cycle in women.

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The menstrual cycle in a woman is necessary for reproduction and typically repeats every 28 days. Onset of the cycle is considered to be on the first day of menstrual bleeding. Menstrual bleeding stops after 3-7 days and a follicle in the ovary that contains an egg begins to develop. Over the next 14 days, estrogen, a female hormone produced by the ovary, begins to increase as other hormones come into play. At mid cycle, about day 14, ovulation occurs when an egg is released from its follicle and begins its journey through the Fallopian tube. The ruptured follicle becomes a corpus luteum and begins producing progesterone, the hormone responsible for causing the lining of the uterus to prepare for the fertilized egg to implant and begin a pregnancy. If the egg is not fertilized by a sperm within about 24 hours or if a fertilized egg does not implant in the thickened lining of the uterus within two weeks, the cycle begins again with the shedding of the uterine lining (menses).