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Hormones 101: Finally Understand Your Reproductive Hormones

If you’re a woman of reproductive age (and if you’re not on hormonal birth control), your body goes through some drastic cyclical changes in hormones every month. You may be aware of these changes (hello, pimples and cramps and mood swings) or you may have no idea, but either way, they are happening. What’s driving all this? Hormones. A great many different hormones, in fact.

This all may sound pretty complex, and it is, but every woman should be able to understand the basic hormones that drive the major phases of the menstrual cycle. This post will go over what hormones are, and what you need to know about the major ones in relation to the cycle, fertility, and pregnancy.

What are hormones?

Hormones are chemical messengers that are produced by endocrine glands, which include the ovaries, thyroid gland, hypothalamus, and pituitary gland. These messenger molecules travel in the bloodstream and circulate throughout the body to impact processes like menstruation, ovulation, and pregnancy. Most hormones are made from cholesterol.

These molecules can also affect other systems like mood—via hormone signals in the brain—and hunger—via hormone signals in the gut. That’s why symptoms like mood swings, nausea, or weight change are often the first signs of something happening with your hormones.

Gonadotropin-Releasing Hormone (GnRH)

What is the role of GnRH in the menstrual cycle?

The primary control center for reproductive hormone changes is the hypothalamus. The hypothalamus sends signals to the pituitary gland, which then sends signals to the ovaries, which then makes estrogens and progesterone in females. This system is called the hypothalamic-pituitary-gonadal axis (or HPG axis). Men have a version of the HPG axis too, but it regulates testosterone at the end instead of estrogens and progesterone.

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Throughout the cycle, the hypothalamus is constantly taking in information about your body and using it to create a rhythm of GnRH pulses. The frequency (number of pulses) and amplitude (how strong the pulses are) of these GnRH pulses encode information about the menstrual cycle.

During the follicular phase (from day one of your period until you ovulate), GnRH pulses are high frequency and low amplitude, leading to stimulation of follicle-stimulating hormone and luteinizing hormone from the pituitary gland (both hormones discussed below). Once estradiol and progesterone reach a certain threshold, the hypothalamus slows down the GnRH rhythm and the pulses switch to low frequency with high amplitude.

What can GnRH tell you about your fertility?

When the pulses of GnRH are thrown off, ovulation doesn’t happen. For example, hormonal birth control sustains estradiol and/or progesterone at high levels, which essentially tricks the hypothalamus into keeping GnRH pulses low and you don’t ovulate (which means you can’t get pregnant).

If you’re planning on using artificial reproductive methods to support your fertility, your doctor will give you medication that essentially quiets your GnRH levels temporarily so that many follicles can develop (aka “superovulation”) and be collected.

What is the role of GnRH during pregnancy?

GnRH activity remains low during pregnancy and lactation. The placenta creates its own GnRH during pregnancy.

Follicle-stimulating hormone (FSH)

What is the role of FSH in the menstrual cycle?

FSH is the hormone that stimulates multiple follicles in your ovaries to mature during the first half of your cycle, called the follicular phase. Each of these follicles is a fluid-filled sac that contain an immature egg (or oocyte). Eventually, one of these follicles becomes dominant, and begins to produce high levels of estrogen, which is necessary for ovulation to occur.

What can your FSH level tell you about your fertility?

If you’re over 35, then your doctor may test your FSH levels on day three of your cycle. The reason for this test is to check ovarian reserve. Measuring FSH early in the cycle indicates how hard is the system working to stimulate enough follicles that cycle. The higher the FSH level, the harder the system needs to work to make enough, which may mean ovarian reserve is low.

 What is the role of FSH during pregnancy?

Once you’re pregnant, FSH is no longer active and stays quiet until you resume cycling after birth.


Estrogens are one of the most recognizable hormones that you may associate with being a woman. There are three different types of estrogens, and women always have all three of them: estrone, estradiol, and estriol (so yes, it’s actually estrogens, not estrogen). Each one has a different profile:

  • Estrone (abbreviated as E1): the predominant estrogen after menopause
  • Estradiol (abbreviated as E2): the predominant estrogen during reproductive years
  • Estriol (abbreviated as E3): produced at high levels during pregnancy

What is the role of estrogens during the menstrual cycle?

Estradiol is the major hormone during the follicular phase (the first half of your cycle, from day one of your period until ovulation). As the follicles in your ovaries grow, they produce estradiol, which builds up the uterine lining.

Around day 10 of the typical cycle, the dominant follicle starts producing enough estradiol to stimulate a surge in luteinizing hormone (explained more below), which leads to ovulation. Estradiol also triggers changes in vaginal discharge, and is directly responsible for making more fertile cervical mucus, the sperm-friendly fluid that increases odds of fertilization.

What can your estrogen level tell you about your fertility?

If the follicle doesn’t produce enough estradiol, then there isn’t a signal for stimulating luteinizing hormone, which means no ovulation. Family or personal history of ovarian cysts increase the likelihood of low estrogen levels. Over-exercise, undereating, calorie deficit, and physiological stress can also contribute to low estrogen levels, sometimes leading to a condition called hypothalamic amenorrhea. Signs to watch out for are irregular cycles, amenorrhea (not getting your period), and vaginal dryness. If you’re concerned about possibly having low levels of estrogens, talk to your doctor about testing for it.

In the case of PCOS, hormones like estrogen are thrown off balance which can lead to anovulation (or no ovulation) and can be a fertility complication.

What is the role of estrogens during pregnancy?

The placenta produces an incredible amount of estrogens during pregnancy. In fact, from the beginning to the end of pregnancy, levels of estradiol and estrone increase 100-fold and levels of estriol increase 1000-fold! These hormones support the placenta by creating new blood vessels and increasing blood circulation, which helps shuttle nutrients to the baby.

Low estradiol during pregnancy have been linked to pre-eclampsia, and researchers are still working on figuring out exactly why and how this happens.

Luteinizing Hormone

What is the role of luteinizing hormone (LH) during the menstrual cycle?

Towards the end of follicular phase, the dominant follicle produces lots of estradiol. Once estradiol levels reach a certain threshold, it stimulates the release of luteinizing hormone (LH) from the pituitary gland. This triggers the dominant follicle in the ovary to release its egg. After the egg is released, the follicle transforms into the corpus luteum (which is why it’s called luteinizing hormone, and why the next phase of the cycle is called luteal phase). The primary role of the corpus luteum is to secrete progesterone, which we’ll discuss in the next section.

What can your LH levels tell you about your fertility?

Ovulation tests measure LH in your urine, and a LH surge is a pretty good indication that ovulation is happening soon. But, a drawback is that how long the LH surge lasts can vary considerably from woman to woman. So, if you have a short LH surge, it might be more difficult for the tests to catch it. And if you have a long LH surge, you might get several days of positive LH tests before ovulation, making it harder to predict exactly when you will ovulate.

Something interesting about LH: its molecular structure is very similar to hCG, the hormone that confirms pregnancy (explained more below). Because both hormones are so biologically similar, LH tests cannot tell the difference between LH and hCG in your urine. So, if you’ve missed your period and have a positive LH test, it’s possible that you’re pregnant (but it’s still better to use a pregnancy test designed to be sensitive to hCG levels specifically).

What is the role of LH during pregnancy?

Like FSH, LH also doesn’t do much once you’re pregnant.


What is the role of progesterone during the menstrual cycle?

Progesterone is the primary hormone during the luteal phase (from ovulation until your next period begins). After the dominant follicle in the ovary releases an egg, the follicle transforms into the corpus luteum and begins to secrete progesterone. This rapid rise in progesterone sustains the uterine lining that estradiol created earlier in the cycle.

Progesterone is responsible for many of the unpleasant symptoms associated with both the menstrual cycle and pregnancy. Progesterone raises your metabolism, which can make you hungrier and lead to cravings. This increase in metabolism also leads to an increase in basal body temperature.) It relaxes your digestive muscles, which can lead to constipation before your period and during pregnancy. It’s also associated with breast soreness.

If you’re trying to get pregnant, all these normal progesterone symptoms might make you suspect that you’re pregnant. Unfortunately, physical symptoms during the luteal phase are not a reliable indicator of pregnancy, since progesterone levels are high at that point in the cycle whether you are pregnant or not.

If you don’t get pregnant that cycle, the corpus luteum shrinks after about 10 – 16 days. Progesterone levels drop, and the uterine lining can no longer be sustained, and is shed during your period.

If you do get pregnant that cycle, as soon as implantation is complete the fertilized egg begins producing a hormone called hCG. This hormone sends a signal to the corpus luteum to keep producing progesterone (in the medical literature, this is referred to as the “rescue” of the corpus luteum). Since progesterone levels remain high, your uterine lining is not shed and you don’t get your period.

If you don’t ovulate during that cycle, your progesterone levels never rise. In this situation, you may experience breakthrough bleeding instead of a true period.

What can your progesterone level tell you about your fertility?

Without enough progesterone, your luteal phase may be shorter than normal (less than 10 days between ovulation and menstruation), or you may experience spotting before period. There is some controversy around the topic of progesterone and luteal insufficiency, but some doctors and researchers believe it can reduce the chances of pregnancy by interrupting the process of implantation.

If your luteal phase is at least 10 days but you’re still struggling to conceive, then your doctor may want to test your peak progesterone levels. Peak progesterone can indicate that ovulation is normal and progesterone levels are sufficient. But, it’s important to know that progesterone peaks about seven days after ovulation. Based on the conventional 28-day cycle, some doctors assume ovulation happens on day 14 and test progesterone on day 21, which would not be accurate if you ovulate on any other day besides day 14. Tracking your cycles and temperature can help you determine when ovulation happens in your cycle, and you let your doctor know so you can get the most accurate results.

What is the role of progesterone during pregnancy?

The placenta produces an incredible amount of progesterone during pregnancy. Not only does this support growth of the placenta, it also suppresses your immune response so that foreign DNA (hint: your baby) can thrive as well as relaxes the uterine muscles until labor, which may be important for some cases of preterm birth.

Human Chorionic Gonadotropin (hCG)

What is the role of hCG during the menstrual cycle?

Every woman has a tiny amount of the hormone hCG—baseline levels tend to be below 5 mIU/ml. But, it is only made in high quantities by the placenta. So, during in a normal cycle, you wouldn’t expect to see significant levels of hCG.

What can your hCG level tell you about your fertility?

If you’re pregnant, hCG levels will approximately double every 48 hours after implantation. An over-the-counter pregnancy test can detect hCG. The best pregnancy test for early pregnancy detection are the ones that can detect very low levels of hCG.

What is the role of hCG during pregnancy?

After implantation, the placenta produces hCG steadily to protect fetal growth. The downside? It may be the reason for morning sickness.

Something to be aware of with pregnancy tests and hCG levels: it’s possible to get a lighter line or negative pregnancy test a few weeks after a positive pregnancy test. This weird phenomenon, known as “the hook effect”, has to do with the fact that the structure of hCG has multiple variations. Depending on how far along you are and how sensitive the test is for different hCG variants, you could see a negative result but still be pregnant. This is more likely to happen starting from about two weeks after your missed period.

So, try to take a pregnancy test early, and if you see a positive test, follow up with your doctor for a confirmatory blood test (rather than taking more over-the-counter pregnancy tests).

During early pregnancy, slow or erratic rising hCG rising patterns may be a sign of miscarriage or non-viable pregnancy. If hCG patterns look weird—meaning erratic or high levels but no embryo in the ultrasound—these may indicate ectopic pregnancy.

Aarthi Gobinath, PhD

Aarthi Gobinath earned her PhD in neuroscience from the University of British Columbia. Her research covers the ways that stress affects the male and female brain differently.

She tackled the issue of sex bias in research by looking at why standard treatments for depression don't always work in the case of postpartum depression. Her work has been covered by Vice and Massive Science.

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