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The Modern guide to the menstrual cycle

The Modern guide to the menstrual cycle

14 min read

Unless you’re actively trying for kids, you might only think about your menstrual cycle when your period comes around. In some ways, that makes a lot of sense: Periods are the easiest part of the cycle to spot (see what we did there?) without any investigation. Even the way we refer to the menstrual cycle puts periods front and center — menstruation is literally in the name. But there’s so much more going on that’s worth paying attention to, even if it’s not quite as obvious.

Understanding the different phases of the menstrual cycle and knowing where you are in yours can be helpful whether or not conception is on your mind:

  1. Get a heads up when your period’s coming (which makes it easier to plan ahead or stock up on products)
  2. If you are trying to get pregnant, it can clue you in to the days when sex or insemination gives you the best chances
  3. It can give you important details to bring to your healthcare provider when managing your reproductive health and conditions like polycystic ovary syndrome (PCOS) and other similar hormonal imbalances.

To that end, welcome to the Modern guide to the menstrual cycle. Below, you’ll find all the info you need to know what’s going on during your cycle, from beginning to end — and additional context for when you might see changes in your cycle.

The key takeaways

  • Your menstrual cycle kicks off with menstruation (a period) and the follicular phase, followed by ovulation and the luteal phase. The average cycle length can be anywhere from 20 to 40.4 days.
  • Your cycle’s main mission is making conception and pregnancy possible — and hormones like follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrogen, and progesterone are all in flux at different points in your cycle.
  • The average age of menarche (first period) is 12.4, while the average age of menopause (no more periods) is 51.
  • Your cycle can be impacted by age, lifestyle factors, health conditions, and hormonal birth control.
  • Not all mammals have menstrual cycles.
  • You can stay on top of your cycle with apps (we have one), ovulation tests (we've got one of these too!), cervical mucus monitoring, and basal body temperature (BBT) tracking. You can also check in with your fertility hormones on day 3 of your cycle with the Modern Fertility Hormone Test.

The menstrual cycle, from beginning to end

The main purpose of the menstrual cycle is to get all your ducks in a row for a possible pregnancy — the “ducks” in this instance being the ovaries and uterus. The cycle kicks off with a menstrual period. After the last phase ends, the first one begins again, and so on, and so on — that is, until menopause (when cycles come to a complete stop).

While many of us were taught in school that the “average” menstrual cycle length (from one period to the next) is 28 days, the American College of Obstetricians and Gynecologists (ACOG) and the US Office of Women’s Health say that it can last anywhere from 24-38 days. But there can actually be an even wider range: One 98,000-person study found that 95% of their participants have cycles between 20 and 40.4 days in length. However long your cycle is, one thing remains constant: Hormones (little chemical messengers) produced by the brain and reproductive system set off a chain of events that preps the body for pregnancy if an egg is fertilized by sperm.

Keep reading as we walk you through what happens in the ovaries (which develop and house eggs) and the uterus (which provides a nourishing home for a growing fetus) during each phase of the menstrual cycle.

The follicular phase: Period + prep

The follicular phase, aka the proliferative phase, begins on the first day of your period (more on this in the luteal phase section) and ends around day 14, based on the “textbook” 28-day cycle. One study found that the average number of days in the follicular phase is 15.8.

What’s happening in the body during the follicular phase:

  1. The pituitary gland in the brain sends two hormones — follicle-stimulating hormone (FSH) and luteinizing hormone (LH) — through the bloodstream to the ovaries.
  2. FSH and LH trigger about 15-20 eggs to mature. Each egg lives in its own tiny fluid-filled sac, called an ovarian follicle.
  3. The ovaries start producing estrogen, which goes back up to the brain and stops FSH production in its tracks.
  4. In ovulatory cycles, aka ones where ovulation occurs, one ovarian follicle steals the show and keeps maturing (and producing estrogen) — the immature follicles that aren’t developed enough for ovulation fade into the background and die off.
The stages of maturation (aka development) of the ovarian follicles in the ovary.

Meanwhile, the lining of the uterus (in medical-speak: the endometrium) is also keeping busy — this is what the “proliferative” refers to (proliferation = rapidly producing cells or tissue). As levels of estrogen increase, the uterine lining thickens in preparation for implantation. At this time, cervical mucus, or the fluid produced by the cervix, picks up production and it becomes more cloudy and sticky.

High estrogen levels = high sexual desire

Your hormone levels during the follicular phase have another effect outside of follicular and endometrial development: increased sexual desire. During this phase, estrogen is high — which was linked in one 2013 study to feeling more in the mood (wink, wink). Essentially, your body is revving up the (hormone) engine to make fertilization more possible.

Ovulation: The egg goes on its journey

About 14 days into the cycle (based on a 28-day cycle), we enter a new, one-day phase where the ovary releases an egg for possible fertilization: ovulation.

This is a time of *peak* fertility. The five days leading up to and the day of ovulation are considered your “fertile window:” when chances are highest for conception. Why are the days before ovulation part of that window? The egg only has 12-24 hours for fertilization, but sperm can last up to five days (!) in the reproductive tract… so sex or insemination during that 6-day window can mean sperm is there waiting when the egg is released. It increases the likelihood that sperm and egg will meet up at *just* the right time for fertilization to be possible.

During ovulation, your cervical mucus is also at its fertile peak — it’s slippery, clear, and stretchy, and has a similar consistency to a raw egg white. If sperm tries to meet up with the egg at this time, the mucus will help move sperm past the cervix and make conception more likely.

What’s happening in the ovaries during ovulation:

  1. 24-48 hours before ovulation, the rise in estrogen produced by the dominant follicle signals that it’s time to ramp up luteinizing hormone (LH) production in the brain.
  2. The dominant follicle ruptures and releases a mature egg. (Ovulation!)
  3. The little finger-like tendrils at the end of the fallopian tube (called fimbriae) grab the egg and send it on its way through the tube (a journey that lasts about 30 hours).
Ovulation! The dominant follicle ruptures and releases the most mature egg.

Ovulation pain? There’s a (German) word for that

Each cycle, the ovaries alternate which one releases an egg during ovulation. About 1 in 5 people with ovaries experience slight abdominal pain on the side with the ovary that’s ovulating — this is known as mittelschmerz, which is German for “middle pain” (because it happens around the middle of the cycle). Where does this pain come from? There are two possible causes: The follicle growth might stretch the ovary’s surface and cause pain, or the ruptured follicle might irritate the abdominal stomach lining.

The luteal phase: It’s go time

The last phase of the cycle is called the luteal phase (sometimes secretory). The word “luteal” refers to the development of the corpus luteum (Latin for “yellow body”) — a structure that forms in the ovary after ovulation to prep the body for pregnancy, regardless of whether or not fertilization occurs. According to one large study, the average length of this phase is around 13.7 days.

What’s happening in the body during the luteal phase:

  1. The ruptured follicle transforms into the corpus luteum, which produces high amounts of both estrogen and progesterone to thicken the uterine lining and make it a receptive and nourishing home for a fertilized egg.
  2. If sperm fertilizes the released egg in the fallopian tube, the fertilized egg heads toward the uterus and implants into its wall to support embryo development. (This takes place around 8-10 days after fertilization.)
  3. In the event of pregnancy, the corpus luteum keeps on producing progesterone for 10 weeks and the implanted embryo begins producing human chorionic gonadotropin (hCG). (HCG is what pregnancy tests, like ours, use to detect early pregnancy.)
  4. If the egg isn’t fertilized after about 24 hours, the uterus sheds the egg and its excess lining in the form of your period (bleeding that lasts ~2-7 days), and the corpus luteum decays.
After ovulation, the ruptured follicle turns into the corpus luteum. (Since the menstrual cycle repeats, the different follicles will be in various stages in the ovary.)

The 411 on PMS

While most people with ovaries (90%) experience some premenstrual symptoms, like bloating, cramping, or mood changes, others experience a broader range of symptoms — both physical and emotional. Premenstrual syndrome (PMS) typically begins after ovulation and before the period starts. The medical community isn’t sure *exactly* why some people experience PMS symptoms, but it’s likely a combo of factors — ovarian hormones being one of them. We know this in part because symptoms often improve if ovulation is suppressed by hormonal birth control.

The factors impacting the menstrual cycle

Okay! You’re all caught up on the phases of the menstrual cycle. But, since our bodies are all different… what factors can influence the ebbs and flows of our unique cycles?

Like we mentioned at the top of this article, the exact length of the menstrual cycle varies from individual to individual. For most people with ovaries, the luteal phase lasts 14 days — but there’s typically more variety in the follicular phase leading up to ovulation, which often ranges from 10 to 16 days.

What’s way more important than how long or short your cycle is consistency from cycle to cycle. If your cycles are irregular, you may ovulate or get your period at different times than expected — or you may not ovulate or get your period at all some months. Some of the irregularities are a natural part of aging, and some may be the result of taking hormonal birth control or having certain health conditions. (Read this article for more info on irregular cycles.)

Below, we’ll talk about some of the factors that influence your cycle the most.

Age and the cycle

Your reproductive window, or the years in which you’re able to conceive, starts at menarche (the first period) and ends at menopause (when the menstrual cycle comes to a complete stop).

In the US, the average age of menarche is 12.4, while the average age of menopause is 51. Starting your period at a younger age may mean menopause comes at a younger age — and genetics play a role in timing of menarche, perimenopause (when the cycle begins to change leading up to menopause), and menopause.

When you first start getting periods, cycles are typically more irregular, but they become more regular as you get older. Once you’re in perimenopause, typically in your mid-40s (but anytime from your late 30s on is fair game), cycles may become irregular again as you get closer to the onset of menopause. That’s because the ovaries are slowing down production of estrogen and progesterone until they stop producing eggs altogether.

Lifestyle factors and the cycle

Irregularity in your unique menstrual cycle can be caused by several different lifestyle factors:

  • Smoking: In one 1999 study, heavy smoking was linked to 4x the risk of shorter cycles, mostly because of an abbreviated follicular phase — and possible increased risk of anovulation (when ovulation doesn’t happen) and a shorter luteal phase.
  • Exercise level: Intense exercise or rigorous athletic training can interrupt the menstrual cycle as a result of low body fat, high stress, and high energy expenditure. (But it’s not just extreme athletes whose cycles are impacted by exercise.)
  • Stress: High stress levels can cause ovulation and menstruation to stop because of their effect on the hypothalamus — the part of your brain that regulates the hormones in charge of the menstrual cycle.
  • Weight: Low or high body weight, as well as rapid weight loss, can lead to absent periods.
  • Pregnancy or breastfeeding: A missed period is an early sign of pregnancy. Breastfeeding can also delay the return of your period after pregnancy.

Health conditions and the cycle

Certain health conditions can also result in disruptions or changes in the menstrual cycle:

Birth control and the cycle

Reversible (i.e., nonsurgical) birth control methods were designed to place obstacles throughout the menstrual cycle, making conception nearly impossible. In the case of hormonal birth control, the synthetic progestins and estrogen typically block conception with two or more of the following moves: suppressing ovulation, thickening cervical mucus, and thinning uterine lining. The copper IUD, which is non-hormonal, instead works by using copper ions to create a toxic environment for sperm, and to prevent egg implantation and pregnancy. (Take a deep dive into how hormonal BC impacts ovulation and the menstrual cycle.)

Many contraceptives aim to mimic the untouched menstrual cycle, so they include time off from the hormones to cause what’s known as “withdrawal bleeding” when a period would typically occur. The body reacts to the withdrawal of hormones by shedding the uterine lining. But this bleeding isn’t a “true” period because ovulation hasn’t occurred — and because the uterine lining is thinner with most types of birth control, the bleeding is often lighter than a usual period.

You can stop periods entirely by skipping the placebo birth control pills if you’re taking oral contraceptives or skipping the 7-day break when using the vaginal ring. The most common side effect of menstrual suppression is breakthrough bleeding (unexpected bleeding), but that typically lessens over time.

How to track your cycle

Staying on top of where you are in your cycle helps you figure out what part of your cycle will come next — making it easier to prepare for your period, time sex or insemination if you’re trying to conceive, or understand how health conditions or lifestyle factors might be impacting your reproductive health. So… how do you stay on top of your cycle? Keep reading:

  • Tracking apps: Typically, apps count the number of days between the last day you reported being on your period and the first day you report being on your next. Using that info, they can make a prediction of how long your cycle is and when your next period will come. Apps that track your LH levels (like the Modern Fertility App!) take the predictions to the next level by grounding them in your *actual* hormone levels.
  • Ovulation tests: Ovulation tests use biological indicators of ovulation, usually LH levels, to help you figure out where you are in your cycle. Most ovulation tests give you a negative/positive result based on LH averages. The Modern Fertility Ovulation Test, on the other hand, detects LH at low, high, and peak levels — making it easier to track ovulation even if you don’t have “average” hormone levels.
  • Cervical mucus tracking: Since your cervical mucus changes throughout your cycle, you can find clues for where you are in your cycle right in your underwear. At the beginning of your cycle, your menstrual blood will likely cover up the presence of any cervical mucus. As estrogen peaks right before ovulation, the mucus will be cloudier and stickier. Once you’re ovulating, mucus will be slipper, clear, and stretchy, and will resemble a raw egg white. After ovulation, once progesterone levels peak, you’ll stop seeing cervical mucus.
  • Basal body temperature method (BBT): Many people with ovaries track their basal body temperature, or BBT (the body’s temperature at rest), to look for signs of ovulation — though the changes are most clear after ovulation has already happened. ACOG) explains that “the body’s normal temperature increases slightly during ovulation (0.5–1°F) and remains high until the end of the menstrual cycle,” and your fertile window is “the 2–3 days before this increase in temperature.”

Closing the loop: The TL;DR

Throughout the menstrual cycle, there are many hormone-led changes in the reproductive system that make conception and pregnancy possible. The cycle begins with your period and ends with your next one, before repeating the cycle all over again. The first phase, called the follicular or proliferative phase, is focused on maturing the ovarian follicles, getting the uterine lining ready, and making cervical mucus easier for sperm to swim through. Ovulation comes next: This is when the dominant follicle releases an egg through the fallopian tube to meet up with sperm. Finally, the luteal phase (or secretory phase) wraps things up by prepping the uterine lining for embryo implantation — or triggering your period if the egg isn’t fertilized.

After reading this (fairly lengthy) article, you might be wondering what your hormones are up to right about now. Our three at-home tests and free app can help you do just that:

  • The Modern Fertility Hormone Test, which measures the same fertility hormones that a doctor would test in a fertility clinic (for a fraction of the price) and helps you understand how your fertility is changing over time. Our hormone test gives you insight into the number of eggs you have and can help you identify red flag issues — like PCOS or thyroid conditions — that could affect your reproductive health down the line.
  • The Modern Fertility Ovulation Test, which works like magic with our app to help you pinpoint your LH levels to predict your 2 most fertile days. With our test, you’ll get more insight than just a positive or negative result — you’ll be able to see your LH change daily and track low, high, or peak levels. This is key for understanding whether you're in your fertile window and approaching ovulation (when you have the highest chances of pregnancy).
  • The Modern Fertility Pregnancy Test, which is just as accurate from the day of your missed period as leading pregnancy tests — and is also more affordable. It also works like magic with our app.
  • The Modern Fertility App, which helps you find your fertile window more easily by logging and tracking your periods, sex or insemination, and Ovulation Tests. With the app, you'll get a countdown on when to test for pregnancy.

Plus... a little trivia: Not all mammals have menstrual cycles

Since the majority of mammals give birth to live babies (rather than lay eggs), it stands to reason there would be countless other mammals with reproductive cycles like ours, right? Nope. Only a handful of mammals (outside primates) have cycles with menstruation.

Here are a few facts you can use to impress your friends with just how much you know about animals and their cycles:

  • Excluding primates, the list of mammals who menstruate is very short: At least four bat species show signs of menstruation, and there is plenty of evidence proving the elephant shrew also menstruates.
  • In 2016, scientists discovered that the spiny mouse has a human-like menstrual cycle too (the first rodent!). Other rodents have what are called “estrous cycles,” which only last about 4-5 days — hamsters can have up to seven of these a month.
  • Mammals who menstruate have what’s called “spontaneous decidualization” — meaning cellular changes in the uterine lining can occur without anything inducing them. (This might remind you of something we wrote about here.)
  • Mammals without spontaneous decidualization don’t menstruate because they only see changes to their uterine lining when external stimuli are present (read: sex, sperm, or pheromones). No spontaneous decidualization = no thickened uterine lining without sperm = no period.

This article was medically reviewed by Dr. Jenn Conti, MD, MS, MSc.

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Sarah duRivage-Jacobs

Sarah duRivage-Jacobs is a writer and editor at Modern Fertility. She lives with her creamsicle cat, Jasper, in New York City and doesn't believe in the concept of TMI.

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