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6.1: The Menstrual Cycle

Difficulty Level: At Grade Created by: CK-12
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How does the menstrual cycle work?

Every month after menarche, and for approximately the next 40 years, a woman's body prepares itself for possible pregnancy. If no pregnancy occurs, then a woman menstruates. Menstruation signals the beginning of a new monthly cycle of hormones in her reproductive system. What happens? How does it work? This section will help to answer these questions.

Did You Know?

  • Adult males produce millions of new sperm every day.
  • Females are born with all the eggs they need.

The female reproductive system works like a relay. As we discussed in the previous section, under the effect of GnRH from the hypothalamus, the pituitary gland releases gonadotropins (FSH and LH). The ovaries pick up the gonadotropins that cause the eggs in the ovaries to mature. The eggs, in their various phases of maturation in the ovary, release estrogen and progesterone. The pituitary releases more FSH and LH, depending on the levels of estrogen and progesterone in the blood. This complex and sensitive feedback system responds to many environmental influences throughout a woman's reproductive years. For example, too much emotional or physical stress can cause irregular periods. Let's take a closer look at how the menstrual cycle works.

Hormonal Cycle Ovarian Cycle Uterine Cycle

The pituitary releases LH and FSH

The ovary releases estrogen and progesterone.

Eggs mature and every month one (sometimes more) bursts from an ovary and is pushed toward the uterus. Uterine lining thickens and secretes nutrients preparing for the fertilized egg to implant. With no implantation, the lining sheds as menstruation.

Figure 5.1 Menstruation is actually the result of three cycles at work.

Menstruation occurs, on average, every 28 days, although some females have cycles as short as 21 days, others as long as every 40 days. Keep in mind that after menarche, periods may be very irregular for the first year. A woman may skip a few months, or have one right after another. It takes a while for all these hormones to reach the right levels and work together.

The following illustrations show what is happening with each of these cycles. First consider the time frame in which these cycles occur and then the sequence of changes in the ovary, hormone levels, or the uterine lining. Look at the illustrations to get a picture of what happens in a female's body week to week. Then look at the various hormone level graphs to see which hormones are rising or falling. The hormone relay begins with the pituitary hormones, which affect the ovaries. The ovaries in turn release gonadal hormones, which then cause the uterus to prepare to shed its lining. Remember that the menstrual cycle begins on the first day of menstruation.

1. Time Frame

A typical menstrual cycle (of which menstruation is only a part) takes about 28 days. Some people have cycles that are somewhat longer or shorter than this. As in other biological functions, normal people differ from one another.

Day 1 corresponds to the start of the girl's menstrual period. The menstrual flow lasts several days and, after a few more days of milder “spotting,” stops completely.

Figure 5.2 Menstrual cycle timeline in days.

2. Ovarian Cycle

In the ovary, a cluster of the ovarian follicles begins to grow at the start of the cycle, but only one egg reaches full maturity. An ovarian follicle is a small cavity in the ovary that contains a developing egg. The egg bursts out of the wall of the ovary on day 14 during ovulation. This is the most fertile period for a woman-the time when she is most likely to get pregnant. The remaining part of the follicle in the ovary turns into the corpus luteum and then shrinks.

Figure 5.3 Ovarian cycle timeline.

3. Hormonal Cycle: Pituitary Hormones

At the beginning of a menstrual cycle, FSH starts low and then gets higher, drifts slightly down, and then reaches its peak and gradually falls clown. LH starts on a similar course, but then it surges up just before clay 14. It is this surge in LH that triggers ovulation.

Figure 5.4 LH and FSH curves.

Look at the curves for estrogen and progesterone. Notice how they go up and down. You can now put together what happens in the ovary with the changes in levels of hormones, since they depend on each other.

Figure 5.5 Gonadal hormone curves.

At the start of the cycle, the increasing levels of FSH bring about the maturation of the ovarian follicle. The follicular cells in turn produce increasing levels of estrogen. Just before day 14, the sharp increase in LH (and to a lesser extent FSH) causes ovulation. After the ovum leaves the follicle, the remaining cells turn into the corpus luteum (the “yellow body”). This is clue to the effect of LH. The corpus luteum in turn starts producing increasingly higher levels of progesterone (as well as estrogen). Notice that when the gonadal hormones are high, the gonadotropins, FSH and LH, are low (because of negative feedback).

4. Uterine Lining

Finally, let us look at the changes of the lining in the uterus. Under the influence of estrogen, it gets thicker in the period before ovulation. After that, its glands begin to secrete nutrients.

Figure 5.6 Diagram of the uterine lining.

How Thick Is the Uterine Lining? By what percent does the endometrium thicken during the menstrual cycle? The lining grows from .04-.06 inches in thickness to .2-.3 inches in thickness.

All of this is to prepare the uterus for pregnancy. If no pregnancy occurs, the level of gonadal hormones gradually drops. As a result, the lining of the uterus sloughs off and menstruation begins and with it a new cycle begins.

Activity 5-1: How Does the Menstrual Cycle Work?

Menstruation results from three cycles working together, each one influencing the others. In this activity you will explore this interaction and learn what happens at each stage of the menstrual cycle.


  • Activity Report


Step 1 Go back and review Section 5 carefully. If you still have questions about what words mean, now is the time to ask, or find out.

Step 2 Study the drawings on pages 37, 38, and 39 that show what is taking place in the hormonal, ovarian, and menstrual cycles during each phase of menstruation.

Step 3 Describe in your own words how the three cycles work together by answering the questions on your Activity Report. Try to figure out how all the different cycles are related. For example, do you see a connection between changes in the levels of hormones and changes in the ovaries and uterus? Be as specific as you can, indicating on what day of the cycle major changes take place. You will have to read the graphs carefully to do this. For example, if hormone levels are constant and then begin to rise, specify when in the cycle they change.

Step 4 When you think you have described the process accurately, compare your answers with those of several other people in the room. If you need to make changes to your answers or ask more questions, do so before you turn your work in.

The Experience of Menstruation

No matter how much a girl knows or prepares for it, her first menstruation is an important event. This section focuses on experiencing menstruation-what happens and what it's like.

Menstruation is an important life experience, not only for what it represents in the reproductive cycle, but also because it affects a woman's body and feelings, as well as the lives of people around her. Menstruation is an ordinary and normal body function. It is also a private function. We usually do not talk about it in public. But there is nothing secret or shameful about it.

Menstruation, also called a period, is usually not the first sign of puberty. A girl's first period usually comes two years after breast development begins, and a year or so after the peak in her growth spurt. Most girls have plenty of time to learn about menstruation and feel “ready” for it, emotionally and physically. Since a girl cannot tell when she will have her first period, she may warty about it-will it happen at school or when she is out with her friends? Talking with her mother or some other trusted adult ahead of time, and making sure tampons or pads are readily available, may ease her worry.

Figure 5.7

Did You Know?

While women in every culture menstruate, not every woman has the same experience. In some cultures, a woman takes a ritual bath at the end of her period. In other cultures, a girl lives alone in a separate hut during her periods.

What will happen?

For the first year or so, a girl's periods may be quite irregular. One may follow another after 20 days, or 40 days. Eventually, her hormones will settle to a fairly regular cycle and she will menstruate about once a month or about every 28 days. Many physical factors (such as strenuous exercise, malnutrition, illness) or psychological factors (such as emotional upsets) can make periods irregular for a while or stop them altogether.

The most common reason women miss their periods is pregnancy. However, missing her period does not always mean a woman is pregnant. The fear of pregnancy itself may cause a woman to miss her period. If a woman misses her period after having sexual intercourse, it is important to find out as soon as possible if she is pregnant.

Menstrual Flow

Menstruation begins as the lining of the uterus comes off gradually and flows out through the vagina. This menstrual flow is quite slow and gradual. There is no sudden “gushing” out of blood like from a cut. Over the next day or two, the flow will increase a bit, and then over another day or so it will gradually decrease and stop. The amount of blood lost during menstruation may vary quite a bit. The flow is heavier for some girls than others, or from one month to another. Normally, girls lose about three tablespoons of blood. With a good diet including plenty of iron, a mineral required to make red blood cells, the body easily replaces the lost blood. But if bleeding is heavy, or the girl's dietary intake of iron is insufficient, she may develop anemia, which is a condition of insufficient red blood cells. A doctor can easily treat anemia.

“We started dancing again but I couldn't help thinking: Suppose the paper towels aren't enough? Suppose it gets on my skirt and Peter says, “What's that . . . your period?”

-Just as Long as We're Together,

Judy Blume

All that a girl needs to do when menstruating is prevent the menstrual blood from staining her clothes. She can use either a sanitary pad or a tampon. In both cases, instructions for use come in the package. Pads and tampons come in various sizes for different amounts of menstrual flow. They should be changed every 4 or 5 hours when the flow is heaviest; and less often when it is light. Girls can learn more about pads and tampons from their mothers, the school nurse, or health professionals.

Fresh menstrual blood has no odor. It will develop an odor if exposed to air. If a girl changes pads often, she doesn't need to worry about menstrual odors. There is no need to use deodorant tampons because the chemicals they contain may irritate the vagina. A girl can remain fully active during her period, including participation in sports.

  • How can a girl keep herself healthy before and during menstruation?
  • List the factors that may affect the regularity of menstruation.

Figure 5.8 Tampons come in a variety of absorbencies, depending on flow of blood. Tampons are inserted into the vagina to absorb blood and should be changed every 4 hours.

Figure 5.9 Pads come in a variety of shapes and thicknesses, depending on the amount of absorbency needed. A woman peels off the center strip of the back and sticks the pad to underwear.

A rare but important complication associated with tampon use is called toxic shock syndrome. This condition is caused by bacteria that may grow in a tampon soaked with blood. The symptoms of toxic shock syndrome include high fever, vomiting, muscle aches, other flulike symptoms, and a rash that looks like sunburn. If a woman experiences any of these symptoms, she should see a doctor immediately.

Toxic shock syndrome is not associated with the use of sanitary pads. It happens only with tampon use, especially the super-absorbent tampons. It can be avoided by not using super-absorbent tampons, changing tampons frequently, washing hands before inserting the tampon, and using a sanitary pad instead of a tampon at night.

Menstrual Discomfort

Many girls remain perfectly comfortable during their periods. Others feel mild to moderate discomfort. Some may be quite bothered by them, in which case help from the school nurse or a visit to the doctor may be necessary. However, we should not think of menstruation as an illness. It is a normal body function.

What Do You Think

  1. Some judges have acquitted (let go) women accused of violent crimes committed while suffering from severe PMS. This is based on the argument that people under conditions of “diminished responsibility” cannot be held accountable for what they do. Do you agree or not? What are your reasons?
  2. If people cannot be held responsible for their actions during periods of temporary physiological circumstances, should they be allowed to engage in risky activities in which others may be hurt (flying an airplane)? Could this provide excuses for discriminating against women?

Menstrual discomfort takes one of two forms. The first is called menstrual cramps, or dysmenorrhea. Cramps may occur during menstruation and can cause pain in the lower abdomen and the back. There may also be some nausea. A hot water bottle on the abdomen, rest, and drinking warm fluids may help. If cramps are severe, medications from a doctor or pharmacy may help.

A second form of discomfort, called premenstrual tension syndrome (PMS), occurs during the several days before menstruation starts. Some physical symptoms include slight swelling of hands and legs, a bloated feeling in the abdomen, temporary weight gain, and headache. Other symptoms of PMS may be psychological, including moodiness, irritability, anger, trouble concentrating, and lack of energy. This is not to say, however, that “it's all in one's head,” and not a real problem. Although the causes of PMS remain unclear, changing hormone levels are probably the cause. Again, a woman should see a doctor if she experiences any severe symptoms.

Girls: The onset of menstruation is not predictable. What would you do if your period started during school?


Boys: Voice changes arc unpredictable, and sometimes so are erections. What would you do if your voice kept cracking while you were trying to give a presentation in class?

Review Questions

  1. What is the difference between the ovarian cycle and the menstrual cycle?
  2. At the time of ovulation, describe where the menstrual, ovarian, and hormonal cycles are.
  3. When does menstruation usually occur during puberty?
  4. What are the pros and cons of tampon and sanitary pad use?
  5. What common discomforts might a girl experience? Explain.
  6. List the factors that may affect the regularity of menstruation.

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Date Created:
Feb 23, 2012
Last Modified:
Jan 30, 2016
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