How Sex Hormones Create the Menstrual Cycle

This excerpt from The Cycle was written by Groove’s founder Jennifer Aldoretta

The information presented below is at the top of my list of “Important Sympto-thermal Method Stuff.” As a matter of fact, I’d also put it at the top of the “Things Everyone Should Know” list. Understanding how to practice the sympto-thermal method of fertility awareness (STM) is an awesome thing, but a lack of understanding about why the sympto-thermal method is so effective as natural birth control or as tool to get pregnant faster makes you much more likely to slip up.

Even if you have no interest in the sympto-thermal method and simply want to learn about how your cycle works, this section is for you.

The information on this page is something I firmly believe everyone should be taught from a young age. Not only would it make young people who are experiencing menstruation for the first time more comfortable with the changes occurring in their bodies, but it would better prepare them to make informed decisions about their health.

Overview of Female Sex Hormones

Each of the hormones I’ll be discussing in the next few paragraphs plays a crucial role in regulating the menstrual cycle.

Follicle Stimulating Hormone (FSH)

Follicle stimulating hormone, or FSH, is produced by the small pituitary gland I mentioned in the previous section about the female reproductive system and endocrine glands. Based on the name, it’s pretty easy to guess that FSH stimulates follicles. And I’m not talking about the follicles on your head! The pituitary releases FSH in order to stimulate the growth of a few ovarian follicles in preparation for the upcoming ovulation. It does this under the instruction of the hypothalamus, which releases GnRH to trigger pituitary hormone production.

Luteinizng Hormone (LH)

Luteinizing hormone, or LH, is also produced by the pituitary gland in response to the GnRH secreted by the hypothalamus. LH and FSH, like the hypothalamus and pituitary glands, are a great duo. After FSH has initialized the development of several follicles, LH takes over the job and helps these follicles continue their development. LH triggers ovulation to occur, and then after ovulation, its role changes. The follicle that was lucky enough to ovulate its egg — and is, therefore, now just an empty sac — undergoes an incredible transformation. LH causes this empty follicle to “luteinize” and transform into a structure called the corpus luteum, which goes on to produce both of the hormones I will discuss below. This information will be discussed in more detail later in this section.

Estrogen

Estrogen is arguably the most well-known of the female hormones. It is, after all, responsible for some of the observable changes that occur during the cycle. It’s also one of the reasons you’re able to practice the sympto-thermal method. Estrogen is secreted in increasing amounts by the developing ovarian follicles in the time leading up to ovulation.

This estrogen secretion is responsible for the initial thickening of the uterine lining, changes in cervical fluid (cervical mucus) that allow for sperm survival in the acidic vaginal environment, and physical changes to the cervix including its firmness and position. Each of these changes will be discussed in detail in the second half of this section. Estrogen is also produced during the second half of the cycle by the corpus luteum. Its role in this case is to continue the growth of the uterine lining to ready it for possible implantation of an egg.

Progesterone

The corpus luteum secretes a large amount of progesterone following ovulation, although, as I mentioned, it will also produce estrogen. Progesterone is responsible for several things, one of which is the post-ovulatory preparation of the uterine lining for a possible pregnancy. When the corpus luteum was still a developing follicle, it produced estrogen that thickened the lining of the uterus. The progesterone (and estrogen) released by the now corpus luteum causes the lining to engorge with blood and become nutrient-rich in case fertilization occurs.

The second responsibility of this progesterone is to tell the body that ovulation has already taken place. This causes cervical fluid, or cervical mucus, to “dry” and once again create a hostile environment for sperm; it also prevents any other follicles from ovulating for the remainder of the cycle. One incredibly useful consequence of this progesterone secretion is the effect it has on basal body temperature, or the body’s core temperature at rest (measured when you first wake up). The progesterone secreted by the corpus luteum after ovulation causes a sustained rise in basal body temperature that lasts for the rest of the cycle, but we’ll discuss that more in the section about how your hormones manifest as fertility signs.

It is estimated that up to 10% of ovulatory cycles result in the release of multiple eggs [23,24]. However, if multiple ovulations occur, it is generally within a 24-hour period, before the corpus luteum forms and begins secreting estrogen and progesterone.

Hormones and the Menstrual Cycle Phases

Now that I’ve provided a brief description of each of the sex hormones, let’s delve into the menstrual cycle. The menstrual cycle is actually made up of two separate cycles: the ovarian cycle (which involves the ovaries) and the uterine cycle (which involves the uterus).

The ovarian cycle is made up of three phases: (1) the follicular phase, (2) ovulation, and (3) the luteal phase. The uterine cycle is also made up of three phases: (1) menstruation, (2) the proliferative phase, and (3) the secretory phase.

For the sake of simplicity, I will be discussing the cycle in terms of the most prominent phases — menstruation, the follicular phase, ovulation, and the luteal phase — though I will also discuss how the other phases fit into the picture. This section isn’t going to be your run-of-the-mill “this is how the menstrual cycle works” lesson that many of us encountered back in grade school. While those lessons did a great job of making us feel incredibly awkward and uncomfortable, they did nothing to explain why the menstrual cycle occurs.

I remember the question “Why do we have a period?” always being met with the response, “So someday you can have a baby!” But when you’re a nine-year-old who’s trying to navigate a world where fitting in is the only thing that seems to matter, being told that you have a period so that “someday you can have a baby” doesn’t exactly feel relevant.

In a multiethnic study, fewer than 10% of girls involved were ever told that menarche (the very first menstrual period) is a normal event in a girl’s life [25]; this unfortunate fact shows just how lacking sex education really is. Having a rich understanding of how and why we menstruate — beyond for the purpose of reproduction — has far-reaching effects. So play close attention. If the last time you learned about menstruation was in grade school, you’ll be surprised by what you’re about to learn. The menstrual cycle is bloody interesting (and perfect for puns). Period.

1. Menstruation

Menstruation is the first phase we will be discussing, and it’s a phase I’m sure we’re all pretty familiar with. But, shhhhh…don’t talk about it! Menstruation is far from the main event in the cycle, despite what we’ve always been taught. In a nutshell, menstruation is the culmination of several weeks of fluctuating hormone levels, explosive follicle growth, an ovum or two bursting through the ovarian wall, follicle death, and constant brain-ovary chemical communication.

More formally, menstruation is a phase of the uterine cycle and is defined as the shedding of the uterine lining that occurs when a pregnancy has not taken place. It usually lasts between four and seven days.

So just how does your body know you’re not pregnant? When an embryo (or fertilized egg) implants in the uterine lining, it releases a hormone called human chorionic gonadotropin (that’s a hella long name, so let’s just call it hCG). This hCG tells your body not to shed the nutrient-rich lining required for fetal development. If hCG is not detected, your body knows you’re not pregnant and menstruation is imminent.

The four major hormones — FSH, LH, estrogen, and progesterone — begin undergoing changes during menstruation that will result in important events later in the cycle.

2. Follicular Phase

By the time menstruation has started, the follicular phase (which, remember, is part of the ovarian cycle) is already underway. The development of several ovarian follicles began as a result of the FSH released by the pituitary gland during the previous cycle. LH — also produced by the pituitary gland — begins to rise near the end of menstruation. Although this LH helps several follicles develop, only one — and less than 10% of the time, two — will develop fast enough to ovulate.

The follicular phase is named for the rapid development of follicles. As I discussed earlier, the ovarian follicles each contain an oocyte, which is a fancy name for an immature egg. The length of the follicular phase can vary greatly from person to person and from month to month based on a number of factors, but we’ll discuss that more in another section.

As development progresses, new cells form in the follicles that begin releasing estrogen. This estrogen causes the proliferative phase of the uterine cycle, in which the uterine lining begins to thicken. This estrogen also causes cervical fluid (or cervical mucus) to develop “fertile” qualities that are vital for sperm survival and transport. We’ll discuss what constitutes fertile cervical fluid (cervical mucus) when we discuss fertility signs and how to check them.

As ovulation approaches, estrogen production in one (and sometimes two) of the developing follicles will surpass the others, causing this particular follicle to become dominant, meaning that it will be the one to ovulate. When the dominant follicle is “selected” because of its high estrogen production, the other growing follicles cease their development and are reabsorbed by the body. When the estrogen produced by the now dominant follicle reaches a certain threshold, the pituitary gland triggers ovulation by releasing a final surge of LH and FSH.

3. Ovulation

Ovulation — which is the second phase of the ovarian cycle — occurs very shortly after this dramatic LH and FSH surge. As I mentioned earlier, it is possible for multiple follicles to become dominant and ovulate within a 24-hour time period.

During ovulation, the ovum (or mature egg) bursts out of the follicle, through the ovarian wall, and is swept into the fallopian tube by the finger-like fimbriae. If the egg is not fertilized, it will disintegrate within 24 hours [26,27,28,29]. The newly empty follicle is now preparing for a transformation that leads to the final phases of both the ovarian and uterine cycles.

Presence of fertile cervical fluid (cervical mucus) is absolutely vital for the fertilization of an ovulated egg. I’ll discuss what constitutes fertile cervical mucus and how to check cervical mucus in detail in another article. You can also view examples of cervical mucus here.

4. Luteal Phase

The luteal phase is the third and final phase of the ovarian cycle. After ovulation, the large amount of LH that was produced to trigger ovulation causes the eggless follicle to “luteinize” and become the corpus luteum — though it is important to note that this process can begin before ovulation takes place. The corpus luteum is a temporary structure that releases both progesterone and estrogen.

This tells the your body not to ovulate again during the current cycle, dries up fertile cervical fluid (cervical mucus), and causes the secretory phase of the uterine cycle (which prepares the uterine lining for the possible implantation of an embryo). The high levels of progesterone released by the corpus luteum following ovulation cause basal body temperature (BBT) — which is the internal temperature of your body when you first wake up — to increase. I’ll discuss the effect of progesterone on basal body temperature in detail when we discuss fertility signs.

If your body realizes that a pregnancy has not occurred — because it has not detected the hormone hCG that’s released when an embryo implants into the lining of the uterus — the corpus luteum begins to break down, causing progesterone production to decrease. Just before menstruation, the corpus luteum degrades quickly.

This sudden drop in progesterone causes the thick uterine lining to break down, and readies the body for menstruation. The end of the luteal phase also sees a rise in FSH, which recruits new primordial (or immature) follicles to enter the race for the next ovulation. For most people, the luteal phase typically lasts for 12-16 days, and is about the same length each cycle.

1. Menstruation

And here we are back at the beginning!

I know that was a lot to take in, but I hope you agree that this information is incredibly important. It is truly mind-blowing to think about all of the oh-so-subtle changes required to produce each of the cycle phases. If the delicate hormonal balance is not maintained, it will show up as unexpected changes and variations in the cycle; but we’ll get into that more both in the next few sections and when we discuss to how use the sympto-thermal method of fertility awareness to maintain or improve reproductive health.