Menstrual Cycle 101: Your Complete Guide to How Your Cycle Works

Your menstrual cycle is the full sequence of hormonal and physical changes your body runs from the first day of one period to the first day of the next, not just the days you bleed. Your period is simply the part you can see.

Getting to know it is one of the most useful things you can do for your health: it helps you make sense of your periods, spot your own patterns, and notice when something’s changed. And yet most of us were taught remarkably little. Hertility’s own peer-reviewed study of 383,085 UK women found that more than one in five didn’t know their cycle length, rising to one in three of those under 25. We really think that needs to change. Here’s exactly what happens between one period and the next: the phases, the hormones behind them, when ovulation happens, and what counts as normal.

Quick facts:

• Day 1 of your period is day 1 of your menstrual cycle. That cycle runs until the day before your next period begins.
• Your cycle is controlled by changing levels of hormones including oestrogen, progesterone, follicle-stimulating hormone (FSH) and luteinising hormone (LH).
• The menstrual cycle can be divided into the menstrual phase, follicular phase, ovulation and luteal phase.
• Ovulation doesn’t always happen on day 14. Its timing varies from person to person and from cycle to cycle.
• Tracking your cycle can help you understand what’s typical for you and recognise changes that might be worth discussing with a healthcare professional.
• If your periods are irregular, heavy, painful or have changed from what you know as normal, that’s worth understanding rather than putting up with. Hertility’s Advanced Hormone & Fertility Test tests for up to 10 hormones to help you see what’s actually driving your symptoms.

What is the menstrual cycle?

First things first: your period and your menstrual cycle aren’t the same thing. The menstrual cycle is the series of hormonal and physical changes that happen from the first day of one period to the first day of the next. 

So, if your period starts on 1 June and your next one starts on 29 June, you had a 28-day menstrual cycle. The first day you have a full bleed is day 1, spotting doesn’t count. Then you count each day until your next period begins.

Underneath, two processes run in tandem. The ovarian cycle is what’s happening in your ovaries  – follicles develop, an egg may be released at ovulation, and the ovary then prepares for a possible pregnancy. The uterine cycle is what’s happening to your endometrium (your womb lining) – it sheds during your periodand grows back in preparation for an embryo to implant. 

Both are carefully regulated by four cycling hormones – oestrogen, progesterone, luteinising hormone (LH) and follicle-stimulating hormone (FSH) and, as you’ll see, they map neatly onto each other. It sounds complex, but it’s far easier to follow phase by phase.

How long is a normal menstrual cycle length?

Thinking that every menstrual cycle is 28 days can be misleading for many women. For adults, a cycle of roughly 21-35 days is within the expected range, so 28 days can be perfectly normal, but so can 24, or 31. In fact, a study examining over 600,000 cycles found that only 13% of cycles in the study group were 28-days long – 29.3 days was the average. 

Rather than expecting your to body fit a textbook number, it’s far more useful to know what’s usual for you. Further, a little month-to-month variation is common. Illness, stress, changes in exercise, energy intake and weight, and different life stages can all make it a little longer or shorter than expected.

A ‘regular’ cycle doesn’t mean your period lands on the same date every month, just that it tends to follow a reasonably predictable pattern. And there’s a real difference between feeling regular and actually knowing your numbers: in Hertility’s research, nearly 5% of women who described their cycles as regular and gave a cycle length were, in fact, reporting one outside the 21-35-day range. Which is exactly why knowing your actual figures beats a vague ‘regular’ or ‘irregular’.

What counts as an irregular menstrual cycle?

For most adults, cycles that are repeatedly shorter than 21 days or longer than 35 are generally considered irregular, but the numbers aren’t the whole story. If your periods always come every 27-30 days and suddenly start arriving every 40, that change is useful information to know.

Plenty of things can influence your cycle, and an irregular period doesn’t automatically mean something’s wrong. Common reasons include the first few years after periods begin (most women’s menstrual cycles don’t regulate until they are ~25 years old) , pregnancy, breastfeeding, perimenopause and hormonal contraception, as well as stress, major changes in weight or energy intake, and intensive exercise. Underlying causes include thyroid conditions, raised prolactin and PMOS, along with some medications. If your periods suddenly become irregular, stay that way for several cycles, or come with other symptoms, it’s worth understanding why.

If irregular periods, missing periods or symptoms that don’t quite add up sound familiar, our Online Health Assessment is a good first step towards understanding what’s going on.

How long should a period last?

A period usually lasts 2-7 days, with 5 to 6 days being most common. Some people naturally have short, light periods; others bleed for longer or more heavily, and flow is often heavier on some days than others. There’s no single pattern everyone has to match, what’s useful is knowing what’s normal for you, so a sudden change stands out.

What is a normal amount of bleeding during my period?

Rather than chasing an exact millilitre figure, the more useful question is simpler: is your bleeding getting in the way of your life? Signs your periods may be heavy include:

• changing period products very frequently
• bleeding through clothes or bedding
• needing to use two types of protection together
• passing large clots, bigger than a bottle cap
• bleeding for longer than seven days
• missing school, work, exercise, social plans or other activities because of your period

Heavy periods can also lead to iron-deficiency anaemia, which may leave you unusually tired, dizzy or short of breath. NICE defines heavy menstrual bleeding by its impact on your quality of life rather than a strict volume, so if it’s disrupting your everyday life, that alone is reason enough to get it checked. You don’t have to just cope.

What are the phases of the menstrual cycle?

The cycle is usually described in four stages, though they overlap rather than switching cleanly:

1. Menstruation – your period begins and the lining of the uterus is shed.
2. The follicular phase – follicles containing immature eggs develop and oestrogen rises.
3. Ovulation – a mature egg is released from the ovary.
4. The luteal phase – progesterone rises and the uterus prepares for a possible pregnancy.

Biologically, there’s a little overlap between these phases. For example, your period actually happens during the beginning of the follicular phase. But thinking about the cycle in four stages is a useful way to picture what’s happening.

The follicular phase: from your period to ovulation

The follicular phase starts on the first day of your period and ends when you ovulate. For most people, this phase lasts about 10-22 days, but this can vary.

At the start of a new cycle, oestrogen and progesterone are low, so the brain releases gonadotrophin-releasing hormone (GnRH), which signals your pituitary gland (also in the brain) to make follicle-stimulating hormone (FSH).

FSH does exactly what its name suggests: it encourages follicles in your ovaries to grow. Each follicle contains an immature egg. Several follicles may begin developing, but usually one becomes the chosen one and continues to grow while the others stop developing, this is called the dominant follicle and continues towards ovulation.

As this follicle grows, it makes increasing amounts of oestrogen. Oestrogen also helps the lining of your uterus to start growing back after your period and also changes your cervical mucus to support sperm travel.

This is the part of the cycle that varies most in length, and it’s usually why a given month runs long or short, because ovulation came earlier or later than usual. A study found that the follicular phase averaged about 17 days, with wide natural variation, shortening by roughly 0.19 days per year of age between 25 and 45.

Ovulation: when an egg is released

Ovulation is the point in your cycle when a mature egg is released from the ovary – the headline event of the whole cycle. 

As the dominant follicle matures, oestrogen increases, and once it’s high enough it triggers a sharp spike in LH, the LH surge. That surge sets off ovulation, usually around 24-36 hours after it begins (LH peaks about 12 hours before the egg is actually released). The egg then enters the fallopian tube, where it may meet sperm.

But having a period doesn’t guarantee you ovulated that month. A bleed without ovulation is called an anovulatory cycle, more on that shortly. 

Does ovulation always happen on day 14?

No, and this is one of the most common (and most consequential) myths going. Day 14 comes from the textbook 28-day cycle, which most people simply don’t have. 

In reality, ovulation happens roughly 10-16 days before your next period, not on one fixed day. Someone with a 24-day cycle and someone with a 35-day cycle won’t ovulate on the same cycle day and even if yours are usually regular, your ovulation day can still shift month to month. It’s less predictable if your cycles are irregular, you’ve recently started your periods, or you’re entering perimenopause.

How can you tell if you’re ovulating?

There isn’t always an obvious sign, but a few changes can point to it. In the days beforehand, your cervical mucus often turns wetter, clearer, stretchier and more slippery (think raw egg white) as oestrogen rises, an environment that helps sperm survive and travel. Ovulation predictor kits test your urine for the LH surge, so a positive suggests ovulation may be approaching, though it can’t absolutely confirm an egg was released. And basal body temperature rises slightly after ovulation, thanks to progesterone, useful for confirming, in hindsight, that ovulation likely happened, rather than predicting it in advance. Some people also feel a mild one-sided twinge around ovulation; others notice nothing at all. 

Our guide to ovulation and how to track it goes further.

When can you get pregnant during your cycle?

Your fertile window is the six days when pregnancy is most likely: roughly the five days before ovulation, plus ovulation day itself. It’s this long because sperm can survive in the reproductive tract for several days, while the egg can only be fertilised for around 12–24 hours after it’s released, which is why sex in the days before ovulation can still lead to pregnancy. Your window can also shift month to month if your cycle length varies.

That sounds simple written down, but pinpointing it in real life trips a lot of people up. In separate Hertility research involving 97,414 women trying to conceive, more than 41% couldn’t accurately identify their fertile window. 

So if you’ve ever felt unsure, our guide on when to have sex to conceive breaks it down.

Can you have a period without ovulating?

Yes, you can have bleeding that looks like a period without having ovulated. This is called an anovulatory cycle. Normally, ovulation turns the empty follicle into a structure called the corpus luteum, which makes progesterone. If ovulation doesn’t happen, that progesterone rise doesn’t either, but your lining can still build up and eventually shed, causing a bleed.

The odd anovulatory cycle can happen to anyone, and they’re more common after periods first begin and during perimenopause. But if you’re regularly having very irregular or missing periods, it can be a sign you’re not ovulating consistently, something conditions like PMOS (polyendocrine metabolic ovarian syndrome, formerly polycystic ovary syndrome (PCOS)), thyroid problems, raised prolactin and hypothalamic dysfunction can all affect.

The luteal phase: what happens after ovulation?

After ovulation, your cycle moves into the luteal phase, whose job is to prepare your uterus for a possible pregnancy. The follicle that released the egg starts making progesterone. Progesterone maintains and enriches your womb lining so an embryo could implant.

If pregnancy occurs, hormonal signals keep progesterone flowing through early pregnancy. If it doesn’t, the corpus luteum breaks down, progesterone and oestrogen fall, the lining begins to shed, and your next period arrives. Unlike the follicular phase, the luteal phase is fairly consistent, typically around 12-14 days, with only modest natural variation.

Why do you sometimes feel different before your period?

The days before your period are when many people notice premenstrual syndrome (PMS). There’s growing research into how hormonal shifts across the cycle can influence mood, energy and how we experience pain, though the picture is still developing, and how much people notice varies enormously. Common PMS symptoms include:

• mood changes or irritability
• feeling anxious or emotional
• tiredness or trouble sleeping
• bloating or cramping
• breast tenderness
• headaches
• changes in appetite
• skin changes

Not everyone gets PMS, and symptoms can vary a lot from month to month. If they’re severe, follow a clear monthly pattern, and significantly affect your relationships, work or daily life, it’s worth speaking to a clinician, a smaller number of people experience a more severe form called premenstrual dysphoric disorder (PMDD). Keeping a daily symptom diary across at least two cycles helps show whether symptoms consistently track a particular phase.

What happens to the uterus during your cycle?

While your ovaries move through the follicular phase, ovulation and the luteal phase, your uterus runs its own three phases in step, different processes, but kept closely linked by your hormones:

• The menstrual phase is your period. With no pregnancy on the way, falling oestrogen and progesterone cause the endometrium to break down and leave the body as menstrual blood. Day 1 of bleeding is Day 1 of a new cycle.
• The proliferative phase follows, as rising oestrogen rebuilds the lining thicker and richer in blood vessels, ready for a possible pregnancy later in the cycle.
• The secretory phase comes after ovulation, when progesterone makes the lining more receptive to an embryo. If implantation happens, the lining is maintained; if not, hormones fall and the next period begins.

Which hormones control the menstrual cycle?

Four hormones do most of the moment-to-moment heavy lifting, in sequence. FSH (follicle-stimulating hormone) stimulates your follicles to develop in the first half of the cycle. Oestrogen rises as they grow, rebuilds your womb lining and triggers the LH surge. LH (luteinising hormone) spikes sharply just before ovulation, this surge releases the egg. And progesterone rises after ovulation to prepare and maintain the lining for a possible pregnancy.

But they don’t work alone. The whole sequence is set in motion by GnRH (gonadotrophin-releasing hormone) from your brain, which tells your pituitary gland when to release FSH and LH in the first place. And several other hormones can shape your cycle from the sidelines:

• Thyroid hormones – an under- or over-active thyroid (reflected in TSH, FT3 and FT4) can make periods heavier, lighter, or irregular.
• Prolactin – raised levels can suppress ovulation and cause missed or absent periods.
• Androgens, including testosterone – when elevated, as in PMOS, they can disrupt ovulation and cycle regularity.
• Insulin – insulin resistance is closely tied to PMOS and can knock ovulation off track.
• Cortisol, your main stress hormone – sustained stress can delay or suppress ovulation, which is why stress so often shows up as a late or skipped period.

Research has also shown that when progesterone and oestrogen drop before your period, your brain’s serotonin (a mood-regulating neurotransmitter) is also affected. 

Two things worth holding onto. First, hormones don’t follow an identical script in everyone, so it’s worth being wary of claims that everyone should feel more energetic, confident or productive at set points in their cycle. Some people may notice clear cyclical shifts, others may notice very little – both are completely normal. Second, because these hormones are interconnected, a single result rarely tells the full story, they’re best read together with clinical oversight.

That’s the thinking behind Hertility’s Advanced Hormone & Fertility Test, which looks at up to 10 hormones side by side including AMH, FSH, LH, oestradiol, prolactin, testosterone, and your thyroid markers, so you get the whole picture of what’s driving your cycle and symptoms, rather than one piece of it. If your periods are irregular, you’re missing periods or haave symptoms that don’t quite add up, our Online Health Assessment is a good first step towards understanding what’s going on.

Can stress affect your menstrual cycle?

Yes. Stress can disrupt the hormonal signals involved in ovulation, and if ovulation is delayed, your period can be too. More significant physical or emotional stress can sometimes contribute to a missed period altogether. But “it’s probably stress” shouldn’t become a catch-all, if something has changed and stays changed, especially alongside other symptoms, it’s worth looking into properly.

Read more in Hertility’s guide to stress and periods.

How does my menstrual cycle change as I get older?

Your cycle isn’t fixed from your first period to your last. In the first few years after periods start, they’re often unpredictable while your brain and ovaries establish the hormonal communication behind regular ovulation; most people settle into a steadier pattern within about three years. If longer or irregular cycles persist beyond that, it’s worth seeing your GP to rule out a cause such as PMOS.

As you move into your 40s, you may notice cycles getting shorter, or periods turning lighter or heavier, often an early sign of perimenopause, the transition in the final years of fertility before menopause. Hormones fluctuate more, ovulation becomes less consistent, and cycles can shift in either direction. Menopause itself is confirmed retrospectively, once you’ve gone 12 consecutive months without a period (assuming there’s no other reason for them to have stopped). After menopause, ovulation and natural cycles stop.

What happens to your cycle on hormonal contraception?

This is an important distinction, because not everyone who bleeds each month is having a natural cycle. Some hormonal contraceptives change or suppress ovulation, the combined pill, for example, generally prevents it. The bleed you may get in the pill-free interval is a withdrawal bleed, caused by the drop in synthetic hormones, rather than a true period following ovulation. Other methods affect things differently: depending on the method, you might bleed regularly, irregularly or not at all. So if you’re on hormonal contraception, the four-phase cycle described here may not reflect what’s happening hormonally in your body.

How can you track your menstrual cycle?

Tracking your cycle can help you understand what’s normal for your body and recognise when something changes. At its simplest, that might mean recording the first day of each period so you know your cycle length.

You can also track things like:

• how long your period lasts
• how heavy your bleeding is
• pain
• cervical mucus
• PMS symptoms
• mood
• symptoms that repeatedly appear at particular points in your cycle

And this isn’t only useful if you’re trying to conceive.

Knowing your baseline can make it much easier to spot when something changes and give a healthcare professional a clearer picture of what you’ve been experiencing.

Given that one in three women under 25 in Hertility’s study didn’t know their menstrual cycle length, even getting familiar with that one number is a useful place to start.

When should you seek help about your periods?

It’s worth seeking medical advice if:

• your cycles are regularly shorter than 21 days or longer than 35
• your periods last more than 8 days
• your periods come more than 2-3 months apart, or stop unexpectedly
• your normal pattern changes significantly
• you regularly have very heavy bleeding, bleed between periods or after sex
• period pain is affecting work, study, sleep or daily life
• You have irregular periods which come with symptoms like persistent acne, increased facial or body hair, unexplained weight changes or nipple discharge
• you’re trying to conceive and are concerned you may not be ovulating regularly

Periods vary. However, severe pain, heavy bleeding and significant changes shouldn’t be brushed off as something you simply have to live with.

Get to know your cycle with Hertility

Your cycle offers real clues about your reproductive health, and knowing what’s normal for you helps you catch changes sooner, make sense of symptoms, and know when something’s worth investigating.

If you’re dealing with irregular or missing periods, heavy or painful periods, or other changes that could be hormonal, we can help you get to answers built on evidence, not guesswork. Depending on your symptoms, history and goals, Hertility’s Advanced Hormone & Fertility Test looks at up to 10 hormones together to build a clearer picture, because your hormones work as a system, not in isolation. And you won’t be left to decode the results alone, every Advanced test includes a consultation with a Hertility expert to talk you through what your hormones mean and what your next steps could be. 

Not sure where to start? Our Online Health Assessment gives you personalised next steps.

Because knowing your cycle isn’t only useful when you’re trying to conceive, it’s part of understanding your health across your whole reproductive life. Your body shouldn’t be a mystery.

Menstrual cycle FAQs

When Does the Menstrual Cycle Start?

Your menstrual cycle kicks in around puberty and your first period is called menarche. 

Unless you get pregnant or have a health condition which affects your periods, you will continue to experience menstrual cycles throughout your reproductive life until menopause.

The average age of the first period is about 12, while the average age of menopause in the UK is 51. That’s approximately 468 periods you’ll have in your lifetime!

What’s a Normal Cycle Length?

You may have heard that an average menstrual cycle is 28 days, but we now know that the majority of people don’t have a  28 day cycle.

An average menstrual cycle can last between 21 and 35 days and can vary from person to person and sometimes cycle to cycle. 

Having a one-off longer or shorter cycle is usually nothing to be worried about. However, if you find your cycles constantly falling out of this range, it might be your body trying to tell you something is not quite right. 

Your cycle length may also change over time. Usually, your cycle is irregular during the first few years after menarche, becoming regular during your 20s, and then irregular again during perimenopause (the lead-up to menopause). 

What’s a Normal Period Length?

An average period length is 3-5 days, but a normal period is classified as lasting anything between 2 and 7 days, with the heaviest bleeding usually during the first two days.

The length of your periods can vary from cycle to cycle because it’s easily influenced by any factors that impact cycle length. 

Consistently irregular periods are characteristic signs of underlying health conditions such as PMOS (formerly PCOS) , premature ovarian insufficiency (POI) and hormonal imbalances influencing thyroid, oestrogen or testosterone levels. 

Are my periods irregular? 

The best way to figure out if your periods are regular or not is to track your menstrual cycles. You could use an app or simply note it down in a journal. Your periods are defined as irregular when your periods are consistently shorter than 2 days or longer than 7 days.  

Your menstrual cycles are defined as irregular when they are consistently shorter than 21 days or longer than 35 days. Or, the difference between your shortest and longest cycle is longer than 20 days.

Does your menstrual cycle tell you how many eggs you have left?

No. Cycle length says nothing about how many eggs remain. Ovarian reserve is assessed using markers such as anti-Müllerian hormone (AMH) alongside an antral follicle count (AFC) on ultrasound and even these can’t count your remaining eggs or tell you whether you’ll conceive naturally. They’re most useful for estimating how your ovaries might respond to fertility treatment.

Resources:

1. NHS. Periods and fertility in the menstrual cycle. https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/
2. NHS. Periods. https://www.nhs.uk/conditions/periods/
3. Ali Z, Wainwright E, Murugesu S, et al. How well do women understand their menstrual cycles? Insights from 383,085 UK-based women. Reproductive Health. 2026. https://doi.org/10.1186/s12978-026-02275-3
4. Wainwright E, Ali Z, Lawrie L, Getreu N, O’Neill HC. Fertility awareness in 97,414 women trying to conceive. Reproductive Health. 2025;22(1):152. https://doi.org/10.1186/s12978-025-02079-x
5. Bull JR, Rowland SP, Berglund Scherwitzl E, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine. 2019;2:83. https://doi.org/10.1038/s41746-019-0152-7
6. Angela R. Baerwald, Gregg P. Adams, Roger A. Pierson, Ovarian antral folliculogenesis during the human menstrual cycle: a review, Human Reproduction Update, Volume 18, Issue 1, January/February 2012, Pages 73–91, https://doi.org/10.1093/humupd/dmr039 
7. Reed BG, Carr BR. The Normal Menstrual Cycle and the Control of Ovulation. [Updated 2018 Aug 5]. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279054/ 
8. NICE. Heavy menstrual bleeding: assessment and management (NG88).
9. Office on Women’s Health. Your menstrual cycle. https://www.womenshealth.gov/menstrual-cycle/your-menstrual-cycle
10. Wilcox, A. J., Dunson, D., Baird, D. D. (2000). The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study. British Medical Journal, 321(7271):1259-1262.