Lesson Explainer: The Menstrual Cycle Biology • Third Year of Secondary School

In this explainer, we will learn how to describe the roles of hormones in controlling the menstrual cycle and explain how they interact with each other.

Many females from different mammalian species experience rhythmic changes in their reproductive system, which allow them to reproduce. Did you know that menstruation, which involves shedding part of the lining of the uterus, or womb, is fairly rare even among mammals? Primates, like humans, great apes, and even some bats and elephant shrews menstruate periodically, but very few other animals do. In all of these menstruating species, menstruation tends to be controlled by the female’s body releasing progesterone. Amazingly, the cycle length is even fairly similar.

The main purpose of the reproductive system is to produce offspring. A woman can become pregnant and produce offspring only during certain years of her life, sometimes called her reproductive years. The menstrual cycle is roughly a monthly cycle that prepares the female body for a possible pregnancy. According to the UK National Health Service, the menstrual cycle begins at puberty at an average age of 12 and lasts approximately 38–43 years until menopause, which occurs on average between the ages of 50 and 55. It is important to note, however, that these values are specific to the United Kingdom and that the such duration may vary between countries. If a woman becomes pregnant during this time, the menstrual cycle stops for around 9 months, before starting again.

To understand the process of the menstrual cycle, let’s first look at the organs it involves. The diagram in Figure 1 shows the main structures of the female reproductive system involved in the menstrual cycle.

The female reproductive system contains two ovaries, which are small oval-shaped structures each about the size of a large grape. The ovaries are responsible for releasing egg cells and certain hormones involved in the menstrual cycle.

Within the ovaries are follicles, each of which contains an immature egg cell, sometimes called an ovum or oocyte. Approximately once every month, one of the ovaries releases an egg cell in preparation for a possible pregnancy. This process is called ovulation. The egg cell travels along its corresponding fallopian tube away from the ovary and toward the uterus.

The uterus is a hollow organ in the female’s pelvic region. Its wall contains smooth muscles that allow it to contract, as well as blood vessels called uterine arteries that provide the uterus with a good blood supply. If a sperm fertilizes the egg cell in the fallopian tube, this forms a zygote. Once this single-celled zygote begins to divide, it eventually becomes an embryo. In preparation for the possible implantation of this embryo and, hence, pregnancy, the uterus develops a thick lining. At the base of the uterus is a tissue called the cervix, which connects the uterus to the vagina.

If the egg cell is not fertilized within 1–2 days of its release, it starts to break down. When no embryo implants in the uterus, the uterine lining, sometimes called the endometrium, eventually breaks down too. The uterine lining then exits through the vagina along with blood and the unfertilized egg cell, in a process called menstruation, commonly referred to as the period. The lining of the uterus then starts building up again and the cycle continues.

The different stages of the menstrual cycle are controlled by different hormones. Let’s look at the effects of these hormones on the menstrual cycle.

Hormones are chemical messengers that are released by endocrine glands directly into the bloodstream. The blood transports these hormones to their target cells and tissues on which they have an effect.

There are four main hormones that control the menstrual cycle.

These four hormones act in a loop that controls the different stages of the menstrual cycle, an overview of which can be seen in Figure 2 below.

As we can see in Figure 2, two of these hormones, the follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are usually referred to as gonadotropins, are released from an endocrine gland at the base of the brain called the anterior pituitary gland. These hormones stimulate the production of another two hormones by the ovaries, estrogen and progesterone, which in turn act back on the pituitary gland, regulating its secretion of FSH and LH.

Although the main organs on which these four hormones act are in the female reproductive system, as they are carried by the bloodstream, they can have an effect on other organs and glands in the body too. The word stimulate means “to increase or encourage a process.” A hormone is said to stimulate a gland if it results in the production of more hormones.

The word inhibit means “preventing or entirely stopping a process.” A hormone is said to inhibit a gland if it results in a reduction in the release of its hormones.

Now, with the help of Figure 3, let’s see how these four hormones work together to regulate the menstrual cycle.

First, the anterior pituitary gland releases FSH, which stimulates follicles in the ovaries, causing an immature egg cell to grow and stimulate the ovaries to synthesize and produce estrogen.

Estrogen causes the uterine lining to thicken, and it also has an effect on the production of other hormones. It inhibits the secretion of FSH and stimulates the secretion of LH from the anterior pituitary gland prior to ovulation.

LH stimulates ovulation, which is when the ovary releases an egg cell from one of the follicles into the fallopian tube, where it may, or may not, be fertilized by a sperm cell.

An empty follicle remains after ovulation, which transforms into a structure called the corpus luteum. This corpus luteum produces progesterone, which maintains the lining of the uterus for the possible implantation of an embryo. Progesterone also plays a role in inhibiting the release of both FSH and LH.

Example 1: Describing the Interaction of Hormones in the Menstrual Cycle

The four key hormones involved in regulating the menstrual cycle interact with each other, either by stimulating (encouraging) the production of other hormones or by inhibiting (stopping) it.

Complete the following sentences using either “stimulates” or “inhibits”:

1. FSH the production of estrogen.
2. Estrogen the release of FSH.
3. Estrogen the production of LH immediately prior to ovulation.
4. Progesterone the production of FSH and LH.

Answer

Generally, the hormones involved in the menstrual cycle interact closely and have a wide range of effects on the female reproductive system as well as on the hormones that are secreted there and elsewhere in the body.

You can see a summary of how these four main hormones interact with each other in the diagram below. The solid line with a black plus sign shows one hormone stimulating the production and secretion of another hormone. The dashed line with a red minus sign indicates one hormone inhibiting the secretion of another hormone.

Estrogen stimulates the production of LH before ovulation, but it inhibits the production of FSH. FSH stimulates the production of estrogen. Progesterone inhibits the production of both FSH and LH.

Therefore, the sentences should include the following words:

1. Stimulates
2. Inhibits
3. Stimulates
4. Inhibits

Let’s look in more detail at how these hormones affect the female reproductive system during the menstrual cycle.

The menstrual cycle occurs over roughly 28 days, although the length of each stage and the duration of the cycle itself differ between women and can change over an individual’s lifetime. Some women may not experience menstruation at all.

Figure 4 shows the main stages of the menstrual cycle and the associated events that occur. You can see how the egg cell is released from the ovary at the top of the graph, the changes in the hormone levels at the middle, and the thickness of the uterine lining at the bottom.

This graph may look confusing now, but let’s use it as an overview before we break it down into separate sections and explore each section in more detail.

The menstrual cycle begins on day 1 until around day 5 with menstruation, commonly called the period. This is when the uterine lining, its blood supply, mucus, and often an unfertilized egg cell are released through the vagina. This occurs as a result of sometimes painful contractions of smooth muscles in the uterine wall, tearing the blood vessels and pushing blood out of the uterus and into the vagina, where it can exit the body.

During this period, the levels of FSH in the blood slowly increase as the anterior pituitary gland releases it in higher concentrations. FSH, you may recall, causes some of the follicles in the ovaries, called Graafian follicles, and the immature egg cells within them to develop, as you can see at the top of the graph in Figure 4.

The next stage of the menstrual cycle occurs approximately between days 6 and 14 and is called the proliferation phase. The levels of FSH rise during this stage, causing one of the follicles in an ovary to mature. This phase is associated with an increase in the level of estrogen, which is released by the developing Graafian follicle. This increase in the level of estrogen stimulates the uterine lining to thicken.

During approximately days 11 to 13, the reproductive system prepares for ovulation. The level of estrogen rapidly increases, causing the lining of the uterus to continue thickening. You may recall that high levels of estrogen inhibit the production of FSH by the anterior pituitary gland, which causes the levels of FSH to drop.

Around halfway through the menstrual cycle, as shown in Figure 4 on day 14, ovulation occurs. The levels of estrogen peak around the time of ovulation, which you may remember stimulates the production of LH and its secretion from the anterior pituitary gland. A spike in the level of LH in the blood causes an egg cell to be released from a Graafian follicle in one of the ovaries, as you can see in Figure 4.

The ruptured Graafian follicle from which the egg cell was released remains in the ovary and transforms into a new structure called the corpus luteum. This corpus luteum releases the hormone progesterone, while the levels of estrogen drop sharply.

The events of days 15 to around 28 differ depending on whether or not an egg cell has been fertilized by a sperm. These events mark the start of a stage called the secretory phase, sometimes called the luteal phase.

If the egg cell is fertilized, it forms a zygote. The secretory phase in this case involves the zygote dividing to eventually form an embryo and this embryo implanting in the wall of the uterus. The corpus luteum in the ovary releases progesterone to maintain a thick uterine lining and inhibit second ovulation by inhibiting the release of LH. The menstrual cycle temporarily stops, and pregnancy begins.

If fertilization, and therefore the implantation of an embryo and subsequent pregnancy, does not occur, the female body prepares for another menstrual cycle around days 24 to 28 toward the end of the secretory phase. The levels of estrogen drop, allowing the levels of FSH to rise again. The corpus luteum in the ovary begins to degrade, as you can see in Figure 4. This breakdown of the corpus luteum means that the levels of progesterone are also dropping, causing the lining of the uterus to begin breaking down. This completes the menstrual cycle and returns it to day 1 with menstruation.

Let’s see on the graph in Figure 5 how the uterine lining changes throughout the menstrual cycle.

The menstrual cycle begins with menstruation between days 1 and 5 as the uterine lining sheds. This appears on the graph as a decrease in the thickness of the uterine lining. You can see that, from day 5 until the start of a new menstrual cycle, the uterine lining rebuilds, so the thickness increases. This prepares the uterus for embryo implantation, if the egg cell released during ovulation around day 14 is fertilized.

Example 3: Interpreting a Graph Outlining the Thickness of the Uterine Lining during the Menstrual Cycle

The diagram provided shows the changes in the lining of the uterus wall over a typical 28-day cycle.

1. What hormone maintains the uterine lining between days 14 and 28?
2. The menstrual cycle begins with the breakdown and shedding of the uterine lining. What scientific term is given to this event?
3. Around day 14, an egg cell ready for fertilization is released from the ovaries. What scientific term is given to this event?

Answer

The menstrual cycle is a rhythmic, roughly 28-day-long cycle that occurs in many women throughout their reproductive years between puberty and menopause. During the menstrual cycle, many changes occur in the female body, particularly in her reproductive system. One change is in the thickness of the uterine lining, shown on the graph in the question.

Let’s look at the key information on the graph before we attempt to answer the questions themselves. The lining of the uterus is initially fairly thick on day 0, but this thickness decreases until approximately day 6. From around day 6 to day 26, the thickness increases again, before beginning to drop at the end of the 28-day cycle.

Part 1

There are four main hormones involved in regulating the menstrual cycle: estrogen, progesterone, FSH, and LH. While the main role of estrogen is to thicken the uterine lining, the role of progesterone is to maintain the thickness of this lining. This means that the uterus becomes prepared for an embryo to implant in it if an egg cell is fertilized by a sperm cell. FSH helps the egg cell and the follicle containing it to develop and mature, whereas LH stimulates the egg cell’s release from the ovary during ovulation.

Therefore, the hormone that maintains the uterine lining is progesterone.

Part 2

The initial stage of the menstrual cycle, from day 0 to around day 6, involves the shedding of the uterine lining. This process is called menstruation, or the period, and involves contractions of the smooth muscles in the uterine lining, breaking and tearing the blood vessels in the uterine wall. These contractions push the lining of the uterus, blood, and the unfertilized egg cell out of the uterus and into the vagina, where they leave the body.

Therefore, the process of breakdown and shedding of the uterine lining is called menstruation.

Part 3

The lining of the uterus is thick on day 14 as this is when an egg cell is likely to be released from the ovary, and the uterus needs to be prepared for the possible implantation of an embryo should the egg cell be fertilized. When an egg cell is released from the ovary, it bursts out of its ovarian follicle in a process called ovulation. The egg cell then travels along the fallopian tube, where it may be fertilized by a sperm cell. If this successfully occurs, the zygote produced eventually develops into an embryo, which can implant in the thick uterine wall.

Therefore, the term given to the release of an egg cell from the ovaries is ovulation.

Now, let’s see how the concentrations of estrogen and progesterone, the ovarian hormones, change throughout the menstrual cycle by looking at their respective levels more closely on the graph in Figure 6.

The levels of estrogen are low during the first 8 days of the menstrual cycle and increase slowly until just before day 14, increasing the thickness of the uterine lining. During ovulation, which you can see on the graph occurring around day 14, the level of estrogen peaks and then falls rapidly. After ovulation, the levels of estrogen slightly increase again, causing the uterine lining to continue thickening, until dropping again after approximately day 22 to prepare for a new menstrual cycle.

The level of progesterone remains low until ovulation, from day 1 to day 14. After ovulation, its concentration increases rapidly to maintain a thick uterine lining until approximately day 20. The level of progesterone remains high until around day 24, at which point it drops to allow the lining of the uterus to break down again and a new menstrual cycle to begin.

Let’s see how the levels of hormones released from the anterior pituitary gland, FSH and LH, change throughout the menstrual cycle by looking at their respective levels more closely on the graph in Figure 7.

Both remain low from day 0 to day 12. From day 12 to day 14, the levels of FSH and particularly LH increase rapidly to promote ovulation on day 14. After ovulation, the levels of both LH and FSH drop and remain low until day 28, but the levels of FSH slightly rise throughout this time.

The levels of FSH increase slowly during menstruation on days 1 to 4, causing the initial increase in the level of estrogen that we see in Figure 6. As we know, estrogen inhibits the release of FSH. So, as the levels of estrogen rise, the levels of FSH decrease again until ovulation on approximately day 14. During ovulation, as the level of estrogen has dropped, the levels of FSH rise again, causing the development and maturation of the egg cell. This increase in the level of FSH causes an increase in the level of estrogen, which, along with high levels of progesterone, causes the levels of FSH to decrease and remain low until the next menstrual cycle begins.

The levels of LH remain very low throughout the whole menstrual cycle, except for a spike during ovulation. The levels of LH spike during ovulation, stimulating the release of an egg cell from a Graafian follicle in one of the ovaries. This is due to the peak in the estrogen levels, as you may remember a high level of estrogen stimulates the production of LH.

Example 4: Interpreting a Graph Outlining the Levels of Ovarian Hormones during the Menstrual Cycle

The diagram provided shows the changes in the hormones produced by the ovaries over a typical 28-day menstrual cycle.

1. What event typically occurs between days 0 and 5?
2. Why do the levels of estrogen peak just before day 14?
   1. To stimulate the production of LH
   2. To release an egg cell from the ovaries (ovulation)
   3. To ensure the uterine lining is ready to be broken down
   4. To stimulate the production of FSH
3. When is progesterone the highest and why?
   1. Between ovulation and menstruation, to maintain the uterine lining
   2. Between menstruation and ovulation, to rebuild the uterine lining
   3. Between ovulation and menstruation, to stimulate the maturation of an egg cell
   4. Between menstruation and ovulation, to stimulate the maturation of an egg cell

Answer

The menstrual cycle is roughly a 28-day cycle that occurs in many human females. This cycle is associated with many changes in the female body, particularly in the reproductive system, that are stimulated by different hormones.

Let’s look at the key information on the graph before we attempt to answer the questions themselves. This graph shows how the blood concentrations of ovarian hormones, estrogen and progesterone, change throughout a typical menstrual cycle. Both remain low from day 0 to day 10. From day 10 to day 13, the levels of estrogen increase rapidly and then drop to promote ovulation around day 14. After ovulation, which is indicated by the dashed vertical line, the levels of progesterone increase until day 20, where they remain high until dropping again from day 24 to day 28.

Part 1

We can see from the graph that the menstrual cycle begins with low levels of estrogen and progesterone in the blood. One of the main roles of estrogen is to thicken the uterine lining, and the role of progesterone is to maintain this thick lining. As not much estrogen or progesterone are present during days 0 to 5, the uterine lining actually decreases in thickness. This process is called menstruation and involves contractions of smooth muscles in the uterine wall pushing the uterine lining, blood, and any unfertilized egg cell out of the uterus, through the vagina, and out of the body.

Therefore, the event that typically occurs between days 0 and 5 is menstruation.

Part 2

Many of the hormones involved in regulating the menstrual cycle interact with each other. Some stimulate or increase the production and secretion of other hormones. Other hormones inhibit or decrease the production and secretion of other hormones.

The hormones involved in the menstrual cycle are estrogen, progesterone, FSH, and LH. Estrogen has a stimulatory effect on LH, which in turn stimulates ovulation. Ovulation occurs around day 14, in the middle of the menstrual cycle. This involves an egg cell being released from one of the ovaries. As you can see from the graph, the levels of estrogen increase rapidly just before ovulation. This stimulates an increase in the production of LH, which stimulates the release of a mature egg cell from the ovary.

Therefore, the levels of estrogen peak just before day 14 to stimulate the production of LH.

Part 3

The main role of progesterone is to maintain the thickness of the uterine lining. You can see from the graph that the levels of progesterone increase rapidly after ovulation, when an egg cell is released from an ovary, around day 14. These levels remain high until around day 26, when they start to decrease again.

Some time is needed after ovulation for a sperm cell to reach and fertilize the egg cell in the fallopian tube. This fertilized egg cell is called a zygote, which then starts to divide to eventually form an embryo. By the time this embryo reaches the uterus, the lining still needs to be thick so the embryo can implant. Progesterone achieves this by maintaining the thickness of the uterine lining.

If the egg cell is not fertilized, the levels of progesterone remain high to keep the uterine lining thick until menstruation starts again and the uterine lining is broken down.

Therefore, the level of progesterone is highest between ovulation and menstruation, to maintain the uterine lining.

Let’s recap some of the key points we have covered in this explainer.