Cheat sheet: How does the menstrual cycle work?

Menstrual cycles. Okay, it’s a pretty detached, sciency term for this incredible thing that continuously happens under the skin of every person with a cycle.

Even though you can’t see them, the hormones at the heart of menstrual cycles affect so much—from how we feel during a month, to when you get we get our periods, when we’re most fertile, to when we feel most in the mood to have sex. Head over to our female sex hormone cheat sheet to find out everything they're up to behind the scenes.

As for menstrual cycles, you may know you have them (or will do one day, or did in the past), but how do they actually work?

The menstrual cycle stages

The menstrual cycle is made up of the ovarian cycle (basically what’s popping in our ovaries) and the uterine cycle (all the happenings in our uteruses). The ovarian and uterine cycle happen simultaneously throughout the month and are each controlled and regulated by our (you guessed it) fabulous, fluctuating, female sex hormones.

Menstruation (~ days 0-5 - bleeding)

What is it?

Got your period? You’re at the start of your cycle. It’s caused by your uterus lining shedding, otherwise known as the endometrium (feel free to slip that into convos with your friends and impress them with your magnificent menstruation knowledge). 

Why does it happen?

If you’re not pregnant, the endometrium that patiently built up throughout the month to support a potential pregnancy isn’t needed. Your body responds by dropping your estrogen and progesterone levels, which triggers your endometrium to shed(2). Until next month, endometrium. 

Where does it happen?

The endometrium sheds in the uterus and is discharged along with blood and any mucus through the vagina. 

Stage 2: Follicular phase (~ days 6-13 - developing)

What is it? 

So you’re done with your period and you’re through to the next stage. This is when your body starts to get another egg ready for fertilization. 

Why does it happen? 

Your pituitary gland (a pea-sized gland in your brain) signals to your ovaries by producing Follicle Stimulating Hormone (FSH)(2). You can read more about FSH here. As the name suggests, FSH has a word with your ovaries, telling them to grow follicles (these are little fluid-filled sacs holding your eggs)(2). About halfway through the follicular phase, there’s a bit of competition in your ovaries. One of your ovaries focuses on developing the largest follicle, which then becomes “the chosen one” to release your egg(3). Kind of like the X-Factor of the egg world. 

Where does it happen?

Follicles develop in the ovaries.

Stage 3: Proliferative phase (~ days 6-13 - rebuilding)

What is it?

When your uterus lining (yep, your good old endometrium) dusts itself off and tries again. Yes, that’s right, it starts building once more. This happens in your uterus.

Why does it happen?

The “chosen” follicle is under the spotlight. It starts growing and producing estrogen, which in turn causes the endometrium to start thickening and building up again(4). 

Where does it happen? 

In the uterus. 

Stage 4: Ovulation (~ day 14 - release day)

What is it? 

If you want to get pregnant, this is the moment you and your body have been waiting for. If you want to avoid pregnancy (and you’re not on contraception or are following natural family planning methods), this is the time to be extra careful…  

Generally, this happens mid-cycle. On average, it’s day 14 for most people but for others who have a longer follicular phase, for example, it could take place later, i.e. on day 16. 

Why does it happen?

Your estrogen levels peak and trigger the release of another hormone called the luteinizing hormone from your pituitary gland (LH)(4). A spike in LH levels signals that it’s time for one of your ovaries to release an egg into the fallopian tube (ovulation)(5). 

Where does it happen? 

An egg is released from the ovaries into the fallopian tube (which is a bit like a runway to the uterus). 

Stage 5:  Luteal phase (~ days 14-28 - transformation)

What is it? 

When your body gets ready for fertilization. 

Why does it happen? 

Once you’ve ovulated, your “chosen” follicle that released the egg transforms into what’s known as the corpus luteum (another turn of phrase to slip into conversation, you’re welcome). This rather seriously-named phenomenon basically refers to a mass of cells that begin producing estrogen and progestogen hormones(6). Estrogen levels begin to fall and progestogen levels peak around halfway through this phase(6). 

Where does it happen?

In that month's particular ovary (they take it in turns). 

Stage 6: Secretory phase (~ days 18-28 - make or break)

What is it? 

This is when your body prepares for pregnancy, or a period. 

Why does it happen? 

When your progesterone level is at its peak, it keeps your built-up endometrium nice and thick so it’s ready to support a fertilized egg (one that has met a sperm)(7). This also triggers the release of chemical messengers which either support(7): 

• The fertilized egg during early pregnancy 

• If the ‘egg meets sperm’ moment doesn’t happen, the corpus luteum dissolves and your estrogen and progesterone levels plummet. Flatlined levels of these hormones kick-start menstruation—and so, the cycle repeats. 

FYI, the global average cycle length is 29.3 days and we’ve included details of average time frames within a natural, regular, menstrual cycle(1). Don’t worry if your cycle doesn’t follow this exact pattern though, it’s totally normal for cycle lengths to vary. 

Where does it happen?

In the uterus.

Menstrual cycle FAQs

Do you still have a female hormone cycle on birth control?

Nope! You won’t have a ‘normal’ hormone cycle with the hormonal fluctuations described above when you’re on hormonal birth control.

The introduction of synthetic hormones (e.g. estrogen and progestin) in the pill, change the levels of the natural estrogen and progestogen hormones produced by your body(8). 

Ovulation usually occurs when your natural levels of estrogen, progesterone, and testosterone are low(2). By introducing synthetic hormones in the pill, these levels cannot fall and so ovulation (the release of an egg) won’t be triggered(9).

A constantly low dose of progestin in the mini-pill/combined pill suppresses the LH surge that triggers ovulation. If ovulation is stopped, this will prevent pregnancy(9).

Is there such a thing as too long or too short for a menstrual cycle?

The ‘textbook’  average menstrual cycle length is 28 days (hence why the regimen of the  first birth control pill mirrored this to please the Pope). However, new research has found that the average actually stands at 29.3 days(1)

A menstrual cycle length that falls in the range of 21-35 days is less common but still considered normal. Cycle lengths can also vary from person to person, i.e. stress can delay your menstrual bleed and therefore increase your cycle length(10). Oh, and your circadian rhythm (natural, internal processes that regulate your sleep and awake cycle) can also change your cycle length, i.e. shift work and traveling(11).

These reasons could lead to your menstrual cycle being less than 21 days(12): 

• Being Perimenopausal (irregular periods are common)

• Stress

• Significant change in weight

• Hormonal birth control (known to change menstrual cycle length)

• Uterine fibroids (noncancerous growths of the uterus)

These reasons could lead to your menstrual cycle length being more than 35 days(12):

• Stress

• Extensive exercise (or being an elite athlete)

• Significant change in weight

• Hormonal birth control (known to change menstrual cycle length)

• Perimenopausal (irregular periods are common)

• Polycystic ovary syndrome (it’s common to experience late periods)

• Pregnancy (of course!)

At which point in your cycle are you most fertile? 

You’re most fertile when you’re close to ovulation. HOWEVER, don’t put all your eggs in one basket (so to speak)—the timing of your fertile window can be highly unpredictable, even if your cycles are usually regular(13).

Your fertile window is usually around 6 days. This window includes the day you ovulate and the 5 days before it(13). If you don’t want to get pregnant, you are advised to not have sex during this time period if you’re not on any type of birth control.

Ovulation takes place around 14 days (2 weeks) before your period starts. However, everyone’s different. This is based on an average 28-day menstrual cycle so it will vary (sometimes hugely) for the individual. 

At which point in your cycle are you least fertile? 

You’re least fertile 5 days after menstruation(13). 

Editor’s note: Throughout this article, we referred a lot to ‘your’. We say ‘your’ body, ‘your’ uterus, ‘your’ eggs. We know everyone is different and these terms may not apply to everyone. We also want to go beyond the science to speak to every one of you in a non-detached, human way. Please get in touch if you’d like to talk this through with the team.

Written by

Hermione Wright (she/her)

An NCTJ-qualified journalist, Hermione writes for national and local publications in addition to creating thought leadership for brands with a purpose. Passionate about telling the stories that matter, she helps our community make their own clued-up choices about their healthcare.

Medically reviewed by

Dr. Alejandra Elder Ontiveros, MD, PHD (she/her)

Ale is a PhysicianScientist with a doctoral degree in Development Biology and Embryology and is currently a Postdoctoral Scholar at UCSF. Ale believes that the union of academia and business can lead to transformative discoveries for women’s health.

Scientifically researched by

Hannah Durrant (she/her)

Hannah is a Biomedical Content Writer at Tuune, with a BSc in Biomedical Sciences from University College London. She is passionate about bringing together the scientific community and the general public by disseminating modern science via digestible, engaging and thought-provoking content.