A woman\’s menstrual cycle is an incredibly complex biological process that enables a woman to be able to carry a pregnancy to term. It is a process that occurs on a monthly basis and prepares the body for pregnancy each month. Researchers have also found that the hormonal cycles which regulate the menstrual cycle are also related to the ability of the body to lay down bone mass density and maintain it through adulthood. This has a significant impact on the development of osteoporosis in later years. (1)
Menstruation occurs only in humans and some mammals such as chimpanzees. Other mammals will go into heat and also bleed but the process is significantly different. The average age of onset of menstruation is age 12. When a woman is counting her cycle it begins on the first day of blood flow. The average duration of a cycle is 28 days but anything between 21 and 34 days is considered normal. For most women the flow lasts for four days but can also last between two and seven days.
On a typical 28 day cycle, ovulation will occur on day 14 and the period begins 14 days after that. The time between the beginning of the period and ovulation is variable but the time span between ovulation and the first day of menstruation is always between 12 and 14 days. It is in the three days preceding ovulation or the day of ovulation that a woman is most likely to get pregnant.
There are four major glands and organs which play an important part in the preparation for menstruation. The first gland is the hypothalamus, located just above the pituitary gland in the brain. The pituitary gland sits above the stalk just behind the eyes and is also important in this process. The ovaries are actually a gland and are about the size of a walnut, sitting directly behind the uterus. The uterus is a muscular cavity, about the size of a pear that sits between the bladder and the rectum.
These glands and organs will secrete five different major hormones that are involved in creating the menstrual cycle. These hormones are gonadotropin releasing hormone, follicle stimulating hormone, luteinizing hormone, estrogen and progesterone.
The first half of the menstrual cycle is the most variable and is known as the follicular phase. During this phase the hypothalamus tells the pituitary gland to produce both follicular stimulating hormone and luteinizing hormone by secreting gonadatropin releasing hormone. Luteinizing hormone stimulates the ovaries to produce estrogen and the follicle stimulating hormone stimulates the production of the eggs.
Approximately s30 different follicle will begin to grow and by day seven the dominant follicle will continue to grow while the others degenerate. This dominant follicle will eventually become the only one ovulating during this cycle. The estrogen secreted by the ovaries will stimulate the uterus to thicken and grow.
By mid-cycle the dominant follicle is producing 500 mcg of estrogen every day to continue to stimulate the uterus in preparation for pregnancy. It is also producing a small amount of progesterone. When the estrogen levels have been this high for two or more days the pituitary gland secretes a large amount of luteinizing hormone which is detectable in the urine and the basis for ovulatory predictor kits. There is also a follicle stimulating hormone surge also. The rapid peak of these two hormones signals the ovary to rupture the follicle and ovulation occurs.
If there is no sperm available the egg travels down the fallopian tubes, through the uterus and is expelled. The second half of this cycle is also known as the luteal phase. During this time the levels of gonadotropin releasing hormone begins to drop and luteinizing hormone begins to rise. High levels of the luteinizing hormone signal corpus luteum (the cysts that was produced when the follicle burst) to produce progesterone. The ovaries produce high levels of progesterone and estrogen and if pregnancy does not occur these levels signal, with the pituitary gland, to turn off follicle stimulating hormone and luteinizing hormone.
Under the influence of high levels of progesterone the uterus stops thickening and starts maturing. If a fertilized egg is present the lining of the uterus is prepared to accept it. If no conception occurs, exactly 14 days after ovulation a period will begin. During menstruation the lining of the uterus is released and comes out as menstrual flow. In effect, the uterus is cleaning itself down to the roots so that the whole process can begin again next month.
The menstrual cycle can be long, short, predictable or unpredictable. In many cases young women and teenagers have unpredictable menstrual cycles. The cycles can change from month to month and over the years. What is normal for one woman may not be normal for another. It is important for a woman to get to know her own cycle by keeping a calendar of events and recognizing how long her menstrual cycles take, how much flow she usually has, and how long she usually bleeds. This information will be an important to note if she believes that she may have a problem with the menstrual cycle or in order to recognize when she may become pregnant.
(1) National Osteoporosis Foundation: Low Bone Density and Osteoporosis in Young Adult Women
Resources
Womenshealth.gov: Menstruation and the Menstrual Cycle Fact Sheet
http://www.nichd.nih.gov/Pages/index.aspx
KidsHealth.org: All About Menstruation