Guest blog by Hertility.
Many of us spend years managing our periods without ever really knowing what "normal" looks like. We figure out what works, get on with it, and quietly wonder whether what we're experiencing is just part of the deal or something worth paying attention to.
You're not alone in that. Hertility's research, the largest UK cohort study of its kind, following 383,085 women found that more than 1 in 5 UK women couldn't report their own menstrual cycle length. Among women under 25, that rose to 1 in 3. These are women already navigating contraception decisions, first reproductive health appointments, and the foundations of their long-term health, often without the basic information they need.
And yet your menstrual cycle is one of your body's most important health signals. The American College of Obstetricians and Gynecologists classifies it as a vital sign, in the same category as your heart rate and blood pressure, because it reflects what's going on with your hormones in real time. A regular, predictable cycle with manageable symptoms is a good sign that things are broadly working as they should. When something changes, or has always been a bit off, it's worth knowing whether that's within the normal range or whether your body is trying to get your attention. This guide is here to help you figure that out.
What does a normal period actually look like?
"Normal" covers a much wider range than the neat 28-day cycle most of us were taught about in school. At Hertility, only 32.4% of our women report a 28-day menstrual cycle. The majority do not and yet 28 days continues to define what most women understand as normal.
Here's what's typical:
Cycle length: 21 - 35 days (counted from the first day of one period to the first day of the next). The average is around 28 days, but anything in this window is normal.
Period length: 2-7 days, with most people bleeding for around 3-5 days.
Flow: Most people lose around 30 - 80ml of blood per period, roughly 2–6 tablespoons. Flow varies naturally from day to day, usually heavier at the start. If you lose more than 80ml per cycle it is considered a heavy period (more on this below).
Colour: Can range from bright red to dark red to brown, and this can change across the days of your period. Brown blood at the start or end is old blood leaving the uterus slowly, and completely normal.
Clots: Small clots are common, especially on heavier days. Larger clots (larger than a bottle cap) passed regularly are worth mentioning to your doctor.
Pain: Some cramping and pain that is manageable with over-the-counter pain relief or at home remedies like a hot water bottle and doesn't significantly disrupt your day is normal. Anything that is causing severe distress and disrupting your routine is not.
All of this can vary slightly from month to month in response to stress, sleep, travel and illness. A bit of variation is fine. It's consistent, significant changes that are worth paying attention to.
If you are worried about any symptoms, talk to an expert.
Period signs that are worth getting checked
The following aren't things you should push through or chalk up to "just how periods are." They're your body's signals, and they deserve to be taken seriously.
Period pain that stops you functioning
Some cramping in the first couple of days is completely normal, it's caused by the uterus contracting to shed its lining. But pain that stops you going to work, school, or getting through your day is not something you should have to accept.
The medical term for painful periods is dysmenorrhea, and it falls into two types. Primary dysmenorrhea is the common cramping caused by the womb contracting to shed its lining, this is what most people experience in the first one or two days of their period. Secondary dysmenorrhea is pain caused by an underlying condition and tends to be more severe and longer-lasting.
Pain that builds with each cycle, isn't touched by standard pain relief, or comes with nausea, vomiting is worth flagging. So is pain that happens during sex, when you use the bathroom, or at other points in your cycle outside of your period. These patterns are among the most common early signs of endometriosis, a condition where tissue similar to the womb lining grows outside the uterus. In the UK, it takes an average of 9 years and 4 months to get an endometriosis diagnosis. That's a long time to be dismissed or to dismiss yourself, and it's exactly why early investigation matters.
Keep a note of when your pain starts, how bad it is out of 10, whether pain relief actually works, and whether it shows up at other times in your cycle. That information makes a real difference when you speak to a doctor.
When the UK Government published its first Women’s Health Strategy in 2022, it was shaped by nearly 100,000 responses from women and people sharing experiences of women’s healthcare. One finding stood out was that 84% said they, or the woman they had in mind, had felt dismissed or unheard by healthcare professionals at some point in their care. At Hertility, our experts are here to listen, take your symptoms seriously, and help you get the care you deserve.
Very heavy periods
Heavy menstrual bleeding, clinically called menorrhagia, affects around 1 in 3 people in the UK. The technical definition is losing more than 80ml per cycle, but the more useful question is simply: is your flow getting in the way of your life?
Your periods may be considered heavy if you experience any of the following:
- Need to change protection every 1 - 2 hours for several hours in a row.
- Use two types of protection at the same time (such as a pad and a tampon).
- Wake up at night to change protection.
- Pass clots larger than a bottle cap regularly.
- Bleed for more than 7 days.
- Bleed so much that it disrupts your sleep, your work, or your ability to leave the house - that needs to be looked into.
Heavy bleeding can sometimes be caused by conditions like uterine fibroids, adenomyosis, or uterine polyps, all of which are treatable once identified. It can also lead to iron-deficiency anaemia over time, which shows up as exhaustion, brain fog, breathlessness, hair loss, and constantly feeling cold. A lot of people live with this for years without ever connecting it to their period.
If any of this sounds familiar, a blood test can check your iron levels and a pelvic ultrasound scan can investigate further the causes of heavy periods.
Periods that have stopped or become very irregular
A one-off late or missed period happens to most people at some point - stress, illness, travel, or a change in routine can all temporarily shift things. That's usually nothing to worry about.
But if your cycle has been irregular for three months or more, or your periods have stopped completely and you're not pregnant, it's really important to get it checked. This is one of the most commonly overlooked signs of a hormonal imbalance and the sooner it's looked into, the sooner you can get the right support.
If your cycle consistently falls outside the 21–35 day window, or the difference between your longest and shortest period is more than 8 days, it would be considered irregular.
Consistently irregular or absent periods, it usually means you're not ovulating regularly. And ovulation matters for more than just having a period, it's essential for getting pregnant. So if you're trying to conceive, or think you might want to in future, irregular ovulation is something to take seriously now rather than later.
Could it be PCOS / PMOS?
If your cycles are unpredictable and you're also noticing acne (especially around the chin and jawline), hair thinning or loss from the scalp, excess hair growth on the face or body, or difficulty managing your weight, PCOS/ PMOS could be behind it. It's one of the most common hormonal conditions, affecting around 1 in 10 people in the UK, and irregular periods are often the very first sign.
Other common causes of irregular or absent periods include:
- Thyroid disorders - an underactive thyroid (hypothyroidism) or an overactive thyroid (hyperthyroidism) can disrupt your cycle, making it irregular or stop altogether.
- Hypothalamic amenorrhoea - a condition where the brain suppresses the hormonal signals needed to trigger ovulation, usually in response to chronic stress, significant under-eating, or high intensity of exercise without enough refuelling. The body essentially puts reproduction on pause when it senses it's under strain.
- Hyperprolactinaemia - prolactin is a hormone produced by the brain that, when elevated, suppresses the signals needed for ovulation. One of the most common causes of high prolactin is chronic stress.
- Perimenopause - the transition to menopause, which can begin in the late 30s or early 40s (the average age of menopause in the UK is 51).
- Significant weight changes - rapid gain or loss can affect the hormones that regulate ovulation.
- Chronic stress - not just emotional stress, but even physical stress from illness, overtraining, or disrupted sleep can disrupt your cycle, making it irregular or stop altogether.
- Travel and sleep disruption - crossing time zones or big changes to your routine can temporarily affect your cycle.
- Illness - even a short-term illness can delay or alter your period.
- Hormonal medications - starting, stopping, or changing hormonal contraception can cause cycle changes for several months. If things haven't settled after 3-6 months, it's worth checking in with your GP.
- Pregnancy or breastfeeding - a missed period is an early sign of pregnancy, and breastfeeding often delays the return of your period postnatally.
- Premature ovarian insufficiency (POI) - when the ovaries stop functioning as expected before age 40. It's relatively uncommon, but important to identify early given its implications for fertility and long-term bone and heart health.
If your periods have been irregular for three months or more, have stopped completely, and you're not pregnant and haven't recently changed contraception, don't put it off. Even if you suspect it might just be stress, it's worth getting checked.
Not sure what's causing your irregular periods?
Hertility can help you get answers and support that goes beyond a diagnosis. Our Advanced At-Home Hormone and Fertility test the key hormones that regulate your cycle and can help identify patterns that might be causing irregular periods with a personalised, doctor-written report and clear next steps. From there, we can support you with a holistic management plan including nutrition and lifestyle guidance, symptom support, and, where appropriate, medication options. By making screening more accessible and management more personalised, we’re helping shift women’s health towards earlier answers, proactive care, and better long-term outcomes.
Bleeding between periods
Any bleeding outside of your regular period is called intermenstrual bleeding and is always important getting looked at.
Light spotting around ovulation (which usually happens around the middle of your cycle) or just before your period starts is usually nothing to worry about. But regular bleeding between periods, bleeding after sex, or multiple days of spotting in the lead up to your period should all be investigated. These can be caused by things like a hormonal shift, a cervical or uterine polyp, the effect of contraception or in some cases an infection.
One thing worth knowing: if you're not up to date with your cervical smear, now is a good time to book one. Intermenstrual bleeding is one of the symptoms that prompts a smear, because it can occasionally be an early sign of cervical changes that are much more manageable when caught early.
See your doctor if you experience:
- Bleeding between periods with no obvious cause.
- Bleeding after sex.
- Any bleeding after the menopause.
Severe mood changes that follow your cycle
Feeling a bit low, emotional, or irritable in the days before your period is something a lot of people experience. But there's a difference between that and symptoms that genuinely take over, where you feel like a different person, your relationships suffer, you can't function at work, or you're overwhelmed by anxiety or depression that seems to come from nowhere.
If that sounds familiar, and those feelings reliably lift the moment your period arrives, it's worth knowing there's a name for it: PMDD (premenstrual dysphoric disorder).
PMDD isn't just a bad version of PMS. It's a recognised clinical condition that affects around 3–8% of people who menstruate and it's significantly underdiagnosed, partly because people assume it's just "how periods are." It isn't. And it has effective treatment options once it's properly identified.
PMDD is often characterised by a shift in emotional and psychological state that can feel difficult to control or explain. Unlike PMS, where symptoms may feel uncomfortable but manageable, PMDD can feel overwhelming and, at times, debilitating.
Many people report intense mood changes, including persistent low mood, anxiety, irritability, or a sense of being emotionally overwhelmed. There can be a loss of interest in things that would usually bring enjoyment, alongside difficulty concentrating or making decisions. For some, these changes are accompanied by intrusive or distressing thoughts, which is why PMDD is recognised as a serious medical condition.
Importantly, these symptoms are cyclical. Symptoms appear in the second half of your cycle, usually the one to two weeks before your period and ease off once bleeding starts. If that's your experience, the most useful thing you can do right now is start tracking it: note your mood each day, when symptoms start, how severe they are, and when they lift. Two to three months of that gives your doctor something concrete to work with and gives you the language to describe what's actually happening, this is also very important for making a diagnosis.
How to track what's normal for you?
Getting to know your own baseline is one of the most useful things you can do, so you notice when something changes. Tracking doesn't need to be complicated. Even just noting the following each cycle is enough:
Your cycle: the date your period starts, and how many days until the next one.
Your flow: record the heaviness of your bleeding. Does it seem lighter or heavier than usual? How often do you need to change your tampon or pad or empty your cup? Have you passed any blood clots?
Clots: yes or no, and how big. A 10p coin or larger than a bottle cap is the benchmark.
Pain: when it starts (before or during your period), how bad it is out of 10, and whether pain relief works. Note if you feel pain at other points in your cycle too.
Other symptoms: spotting between periods, nausea, bloating, skin changes, mood shifts. Note where in your cycle they happen, that's often the most useful part.
Two to three months of this gives a doctor a real picture of what's happening. It can also help you spot patterns you hadn't noticed and make the difference between leaving an appointment with answers versus leaving with "let's wait and see."
Track your cycle and symptoms with the Hertility app.
When should you seek help for your symptoms?
- Your periods suddenly stop for more than 90 days - and you're not pregnant.
- Period pain regularly stops you functioning normally.
- Your periods become irregular after having been regular.
- You bleed for more than seven days.
- You bleed more heavily than usual or soak through more than one pad or tampon every hour or two.
- Your periods are less than 21 days or more than 35 days apart.
- You're spotting between periods or after sex.
- You haven't started your periods by age 16.
- You suddenly get a fever and feel sick after using tampons.
- You're experiencing mood, energy, or physical symptoms that get noticeably worse at the same point in your cycle every month and are affecting your daily life.
- You suspect endometriosis, PCOS/PMOS, or another hormonal condition.
Keeping track of your period is the first step to understanding what's normal for you, and knowing when something isn't. If you have any concerns about your cycle, please don't sit on them. You deserve to have them taken seriously.
Understanding your hormonal health
Your period is the most visible sign of your hormonal health. The hormones that regulate your cycle also affect your energy, mood, skin, fertility, metabolism, and long-term health. When something feels off, understanding which hormones are involved is often the missing piece.
So whether you're trying to make sense of your symptoms or simply want to understand your hormonal health better, Hertility's Advanced Hormone Test checks up to 10 key hormones, including those linked to PCOS/PMOS, thyroid function and your ovarian reserve. You get clinically reviewed results that explain what's happening and why, so you can have more informed conversations with your doctor, and make decisions about your health with confidence.
References
1. American College of Obstetricians and Gynecologists. Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign [cited 2026 May 19]. Available from: https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2015/12/menstruation-in-girls-and-adolescents-using-the-menstrual-cycle-as-a-vital-sign
2. https://www.endometriosis-uk.org/sites/default/files/2025-01/Getting%20a%20diagnosis%20Jan%202025.pdf
3.https://assets.publishing.service.gov.uk/media/61c30d1a8fa8f54c0d15434f/results-of-the-womens-health-lets-talk-about-it-survey.pdf
4. Munro MG, Critchley HOD, Fraser IS, Committee the FMD. The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. International Journal of Gynecology & Obstetrics [cited 2026 May 19];143(3):393–408. Available from: https://onlinelibrary.wiley.com/doi/abs/10.1002/ijgo.12666
5. American College of Obstetricians and Gynecologists. Management of Acute Abnormal Uterine Bleeding in Nonpregnant Reproductive-Aged Women [cited 2026 May 19]. Available from: https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/04/management-of-acute-abnormal-uterine-bleeding-in-nonpregnant-reproductive-aged-women
6. American College of Obstetricians and Gynecologists. Thyroid Disease | ACOG [cited 2026 May 19]. Available from: https://www.acog.org/womens-health/faqs/thyroid-disease
7. https://hertilityhealth.com/blog/menstrual-cycle-basics
8. https://hertilityhealth.com/blog/menstrual-cycle-awareness-uk-study
9. https://www.iapmd.org/united-kingdom
10. https://www.nhs.uk/conditions/endometriosis/
11. https://www.nhs.uk/conditions/heavy-periods/
12. https://www.nhs.uk/conditions/polycystic-ovary-syndrome-pcos/
13. https://www.nhs.uk/conditions/pre-menstrual-syndrome/
14. https://www.nice.org.uk/guidance/ng88
15. https://www.rcog.org.uk/for-the-public/browse-our-patient-information/managing-premenstrual-syndrome-pms/
Blog disclaimer
Our blog is intended to share information and ideas around periods, health, and sustainability. While we do our best to keep content accurate and up to date, things can change over time. The information here is not intended as medical advice — for any health-related concerns, please consult a qualified healthcare professional. For more information on our claims, please see our Claims Page, and for the most up-to-date product information, please visit our Product Pages.
