The menstrual cycle is one of the basics functions of female reproductive health. New World Encyclopedia says menstruation begins at puberty and ends with menopause. The menstrual cycle is the entire, recurring cycle of physiological changes in females that is associated with reproductive fertility, while menstruation is the part of that cycle that involves the shedding of the uterine lining between ovulations.
The average menstrual cycle lasts around 28 days. It occurs in three phases: the follicular phase, the ovulatory phase, and the luteal phase.
The follicular phase starts on the first day of a woman’s period and generally lasts 14 days. The Cleveland Clinic lists the events that occur during this phase:
Two hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH) are released from the brain and travel in the blood to the ovaries. The hormones stimulate the growth of about 15-20 eggs in the ovaries each in its own "shell," called a follicle. FSH and LH also trigger an increase in the production of the female hormone estrogen. As estrogen levels rise, it turns off the production of follicle-stimulating hormone. As the follicular phase progresses, one follicle in one ovary becomes dominant and continues to mature. This dominant follicle suppresses all of the others which stop growing and die. The dominant follicle continues to produce estrogen.
The ovulatory phase is the midpoint of the menstrual cycle, with the next menstrual period starting about two weeks later.
A Suite 101 article says during the ovulatory phase, the dominant egg breaks through its follicle and is released from an ovary, causing a woman to ovulate. This process can take between 16 and 32 hours and is caused by luteinizing hormone.
Cleveland Clinic describes the luteal phase as beginning right after ovulation. At this point, once it releases its egg, the empty follicle develops into a new structure called the corpus luteum. An About.com Pregnancy & Childbirth article says in the luteal phase, progesterone and estrogen help the endometrium, or uterine lining, thicken and create a source of blood flow to accept a fertilized egg. Suite 101 adds it also causes the breasts to swell due to milk duct dilation.
If intercourse has taken place and a man's sperm has fertilized the egg, the Cleveland Clinic says, the fertilized egg will travel through the fallopian tube to implant in the uterus. The woman is now considered pregnant. According to About.com, after about 14 days in the luteal phase, if fertilization does not occur, the corpus luteum withers causing a drop in the estrogen and progesterone, signaling the period to begin again.
Sources:
The Female Reproductive System. The Cleveland Clinic. Web 16, Aug 2011.
http://www.cchs.net/health/health-info/docs/2400/2418.asp?index=9118
New World Encyclopedia: Menstruation. Web 16 Aug 2011.
http://www.newworldencyclopedia.org/entry/Menstrual_cycle
Menstrual Cycle Basics - Your Period. About.com. Pregnancy & Childbirth. Web 16, Aug 2011.
http://pregnancy.about.com/od/gettingpregnant/qt/menstrualcycle.htm
The Phases of the Menstrual Cycle. From Preparation to Ovulation to Menstruation. Women’s Health Suite 101. Web 16, Aug 2011.
http://www.suite101.com/content/the-phases-of-the-menstrual-cycle-a43331
Reviewed August 16, 2011
by Michele Blacksberg R.N.
Edited by Jody Smith