The menstrual cycle is an approximately 28-day cycle during which the ultimate result of the cycle is 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. This latter possibility takes place as the monthly bleeding called menstruation and is considered as "day one" of the 28-day cycle. This delicate cycle results because of a complicated interplay among several hormones.
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. According to Dr. Corrine Welt, writing in the medical database UpToDate, during this phase, called the follicular phase of the menstrual cycle, an increase of 30% in the amount of FSH occurs. This increase stimulates the growth and development of new follicles, one of which will become the "chosen" follicle and develop into the ovulated egg.
Estrogen
Estrogen proves 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, which is 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--at this point, 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. According to Dr. William Ganong, writing in the medical text "Current Diagnosis and Treatment: Obstetrics and Gynecology," ovulation generally occurs within 9 hours of the LH surge. The egg releases from the ovary and becomes 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.
Progesterone
Once ovulation has occurred, the hormone progesterone releases from a structure called corpus luteum. One of the functions of progesterone includes helping to make the mucus around the entrance of the uterus--cervix--thick and sticky. Another function includes helping thicken the uterus wall, so that it prepares 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.
References
- "Current Diagnosis and Treatment: Obstetrics & Gynecology"; Alan H. DeCherney and Lauren Nathan; 2007
- "UpToDate"; Denise S. Basow; 2010
