Your Menstrual Cycle: How the Female Reproductive System Really Works

Long Before Menstruation Begins

Baby girl with pink towel
When a baby girl is born, she has two million eggs in her ovaries.. Photo: tec_estromberg / flickr

The female reproductive system is highly complex, and for everything to work just right, several hormones, glands, and organs must do their jobs in the right order, at the right times. Once you understand how the reproductive system works, it's easier to understand how things can go wrong. It's quite a miracle that things go right at all, given how complicated the entire system is!

Also, being familiar with how the reproductive system works can help you understand why certain fertility treatments are done at specific times.

Familiarity with the reproductive system may even help you understand how to get pregnant faster, by helping you time sex for ovulation.

Long Before Menstruation Begins

We often think of the reproductive system as something that works in monthly cycles, just like our menstrual cycle. While this is mostly true, it's much more complicated than that.

The egg you may ovulate this month has been getting ready inside a follicle for the past 290 days, almost 9 months. In fact, that egg has been around as an immature egg since you were just a fetus inside your mother's uterus.

When you were just a wee one, at about 12 weeks gestation, your ovaries contained 6 to 7 million immature eggs –- the most you would ever have in your life. By the time you were born, there were only 2 million immature eggs left, and by the time you had your first menstrual cycle, your ovaries held just 500,000.

What happens to all those millions of potential eggs? Where do they go?

The Great Egg Race: How Follicles Mature to an Ovulated Egg

Microscoptic image of primary follicle in ovary
This is a microscopic image of a primary follicle. Primary follicles become secondary follicles, and then, eventually, an egg (or eggs, in the case of twins) will ovulate from them.. Photo: Jpogi / Wikicommons / CC BY

Those millions of immature eggs that were contained in your ovaries when you were young are contained in what are known as primordial follicles, and many of them die off as time passes.

When it came time for your first menstrual cycle, the first group of some of the surviving dormant follicles began to "wake up".

As they wake up, there is a sort of competition between them as they grow.

Each month some of these primordial follicles stop growing, with only the best of the group moving onto the next month of growth. Of this group, only a select group will go onto becoming primary follicles, and then onto becoming what is known as secondary follicles.

It's a bit like a follicle marathon to see who will get to become the winning egg.

Eventually, only one (and sometimes two) of these follicles become a mature egg to be ovulated.

But what causes these follicles to race in the first place?

Getting the Great Egg Race Started

The hypothalamus and pituitary gland are responsible for the regulation of many hormones.
The hypothalamus and pituitary gland are responsible for the regulation of many hormones, including GnRH, FSH and LH.. Photo © A.D.A.M.

Getting the follicle-to-egg marathon running is a bit like a relay race in and of itself.

Until the primordial follicles become primary and then secondary follicles, the race primarily takes place inside the ovary on its own.

But when it comes time for a group of secondary follicles to sprint to becoming the ovulated egg, hormones from outside the ovary play a more important role.

The hypothalamus gland, which is located in the brain, starts the relay race off by releasing gonadotropin-releasing hormone, or GnRH. This happens on the first day of your period.

This hormone signals the pituitary gland to generate two more hormones, follicle-stimulating hormone, or FSH, and luteinizing hormone, or LH.

While the pituitary gland is producing both FSH and LH, it stores most of the LH for later. During the first few days of your menstrual cycle, it's the FSH that gets released the most.

This is the beginning of what is known as the follicular phase of the menstrual cycle.

The Follicular Phase of the Menstrual Cycle

Hormone interaction of FSH, estradiol, LH, and progesterone
When estrogen levels reach a certain point, FSH and LH surge, leading to ovulation.. Speck Made / Wikicommons / CC BY / Altered

During the follicular phase of the menstrual cycle, about five to seven follicles in your ovary (and sometimes both ovaries) start racing towards the finish line.

Their growth is encouraged by the FSH hormone. The name gives it away –- FSH is follicle-stimulating hormone, or in other words, the hormone which stimulates the follicles to grow.

As the follicles grow larger, they begin to release the hormone estrogen. As this estrogen travels through the bloodstream, it makes its way back to the pituitary gland, causing the gland to decrease the FSH production.

This is called negative-feedback – as estrogen rises, FSH lowers.

Eventually, one (and sometimes two) of the follicles become a dominant follicle. The dominant follicle releases an even greater amount of estrogen into the bloodstream.

When the follicle reaches the final stages of maturity, the negative-feedback cycle switches to a positive-feedback cycle. This means that rising estrogen leads to rising FSH.

In other words, the high levels of estrogen suddenly cause a spike in FSH, kind of like a last jolt to the maturing egg.

But after this last FSH sprint, the pituitary gland abruptly slows down the production of FSH.

This is the beginning of the next phase of the menstrual cycle, known as the ovulatory phase.

The Ovulatory Phase of the Menstrual Cycle

Interaction between body basal temperature chart, changing hormones, and egg development and ovulation
When LH surges, you're about to ovulate. After ovulation, the hormone progesterone rises, which raises your body basal temperature.. Image: Isometrik / Wikicommons / CC BY

As the levels of FSH lower, the dominant follicle continues to mature and release estrogen. The other follicles who did not win the race die off.

Now, it's only about the winning egg or eggs (in the case of un-identical twins).

The estrogen released by the mature follicle does more than just shut off the supply of FSH from the pituitary. The estrogen is also responsible for stimulating the endometrium, or lining of the uterus, to grow.

The endometrium grows from about 0.5 millimeters in thickness at the beginning of your cycle, to 5.5 millimeters in thickness by the end.

The estrogen hormones are also responsible for the increase in fertile cervical mucus and changes in your cervical position, preparing conditions that will be just right for transport and acceptance of sperm to the egg. It also increases your level of sexual desire, biology's way of helping you time sex for pregnancy.

When the dominant follicle is ready to be released, the levels of estrogen peak. This peak causes the pituitary gland to be extra sensitive to the hormone GnRH, leading to the release of the stored up LH in the pituitary gland.

This surge in LH signals the follicle to release the egg. On the day of your LH surge, you'll have the most fertile cervical mucus and your cervical position will be high, with a soft and open cervix. It's this surge that ovulation predictor kits help you detect.

Within 24 to 48 hours of the LH surge, the dominant follicle ruptures and the winning egg is finally released in a process known as ovulation.

The Luteal Phase of the Menstrual Cycle

Illustration of ovulation and fertilization in female reproductive system
The ova is released and travels through the fallopian tube. In the fallopian tube, if there are sperm present, the egg may become fertilized.. Image: BruceBlaus / Wikicommons / CC BY

After the egg is released, the luteal phase of the menstrual cycle begins.

The released egg has about 24 hours to get fertilized by waiting sperm, and this usually occurs just as the egg enters the fallopian tube. If you had had sex on the days of the LH surge, sperm should be there, waiting to greet the ovulated egg.

In the meantime, the LH hormone causes the ruptured follicle to become what is known as a corpus luteum. The corpus luteum's job is to keep releasing estrogen and to release a new hormone, progesterone.

Estrogen encourages the lining of the uterus to keep growing, while the hormone progesterone helps the lining of the uterus become receptive to the fertilized egg.

Progesterone is responsible for those imaginary pregnancy symptoms that torture many of us during the two week wait.

The hormone progesterone also causes a slight increase in your body temperature, which is what causes the temperature shift you see if you're charting your basal body temperature.

The corpus luteum's life is short, however.

If an egg is fertilized, the embryo will release the hormone hCG, or human chorionic gonadotropin. hCG is very similar to the hormone LH, and it keeps the corpus luteum alive, producing more estrogen and progesterone to maintain the pregnancy.

But, if the egg did not become fertilized, then the corpus luteum begins to disintegrate about three days before you get your period. The estrogen and progesterone hormone levels drop, causing the endometrium to break down and eventually lead to menstruation.

While we are crying about another unsuccessful month, however, our bodies are wasting no time with pity parties. The day your period begins is the day that the hypothalamus begins releasing GnRH once again, starting off another marathon for the next group of waiting follicles.

Fertility Treatment and Your Menstrual Cycle

Gonadotropins with syringe
Gonadotropins like FSH and LH are biological identical to the hormones the body produces to stimulate ovulation.. AWelshLad / iStock

Now that you understand the female reproductive system, it'll be easier to understand the how and why of medicated cycles.

For example, you now know that it takes quite some time -- over 9 months -- for an immature follicle to get ready to race towards ovulation. This is why some lifestyle changes can take time to make a difference in your fertility.

Also, drugs like metformin, an insulin resistance drug that is sometimes used to treat infertility in women with PCOS, can take up to 6 months to work.

Fertility Drugs

You can also better understand why you take certain fertility drugs at particular times during your cycle. For example, Clomid is taken during the early part of the follicular phase of your cycle, because that is the time when the follicles are growing and preparing for ovulation.

During an IVF cycle, you may take what is known as a GnRH antagonist for a week before you expect your period. The GnRH antagonist prevents your pituitary gland from releasing LH and FSH, so that your doctor can control the cycle.

Drugs like Gonal-F and Follistim are made of FSH hormones. You now know that FSH is the hormone responsible for stimulating the growth of the follicles in your ovaries.

Drugs like Ovidrel, often referred to as the "trigger shot", replace or boost the LH surge which triggers the final maturation of the egg and eventually ovulation.

And after ovulation, progesterone supplements may be given. During IVF, when the eggs are retrieved, the follicle is also removed, which means there's no corpus luteum left behind to produce the progesterone required to support the uterine lining for potential pregnancy. That's why you need to take progesterone during IVF treatment.

A Delicate Balance

Microscopic image of primary follicle with egg / ova inside
The reproductive systems of men and women -- from the development of ovum to the formation of sperm -- are amazing and beautiful.. Photo: Ed Uthman / flickr / CC BY

Given the complexity of the reproductive system, it's a miracle that it works as often as it does. Plus, we have not spoken about the male side of the equation.

We also have not even begun to address how other hormones in the body may affect the reproductive hormones, or how age, weight, and other factors can impact this delicate system.

When you really look at it, it's quite amazing, and for me, inspires a deep sense of awe in the world we live in and the things that many of us take for granted.

Learn more your reproductive system:


Falker, Elizabeth Swire. (2004). The Infertility Survival Handbook. United States of America: Riverhead Books.

Greene, Robert A. and Tarken, Laurie. (2008). Perfect Hormone Balance For Fertility. United States of America: Three Rivers Press.

Speroff, L, Glass, RH, Kase, NG. (1999) Clinical Gynecologic Endocrinology and Infertility, 6th edition. United States of America: Lippincott Williams & Wilkins.

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