The menstrual cycle is an approximately 28-day cycle which results in the release of a mature egg from the ovary. This egg may then go on to become fertilized or may be released, unfertilized, along with the lining of the uterus. The latter may take place with monthly bleeding called menstruation -- day 1 of the 28-day cycle. This delicate cycle results from a complicated interplay among several hormones.
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Follicle-Stimulating Hormone (FSH)
As the name implies, this hormone stimulates the development of new follicles as well as the production of the hormone estrogen. During this phase, called the follicular phase of the menstrual cycle, an increase in FSH occurs. This increase stimulates the growth and development of new follicles, one of which will develop into the ovulated egg.
Estrogen is responsible for the continuing development of follicles within the ovaries. However, the effects of estrogen are not limited to within the ovaries. In the uterus, the rising levels of this hormone play an important role in thickening the endometrium -- a layer of the uterus. It also causes the mucus within the cervix to become thicker. Finally, estrogen release acts as a suppressor of its own release -- called a negative feedback loop. It also acts to suppress the production of LH, until just before ovulation. Afterward, estrogen actually stimulates the release of large amounts of LH in what is called the mid-cycle LH surge.
Luteinizing Hormone (LH)
LH peaks in the middle of the 28-day cycle. This is typically called the LH surge and serves as a signal that ovulation -- the release of the mature egg from one of the two ovaries -- is about to occur. During this peak of LH release, concentration of this hormone becomes ten times higher than usual. Ovulation generally occurs within 9 hours of the LH surge. The egg releases from the ovary, able to be fertilized for about 1-2 days after it releases. If it does not become fertilized, it begins to disintegrate or releases along with the inner lining of the uterus as part of the monthly menstruation cycle.
Once ovulation has occurred, the hormone progesterone releases from a structure called corpus luteum. Progesterone makes the mucus around the entrance of the uterus thick and sticky, preparing for a potential pregnancy. If the released egg becomes fertilized, it will become implanted in the wall of the uterus and the fetus will begin to grow.