Irregular periods perimenopause brings are the first reliable sign of the transition. Cycles that vary by seven days or more, then a skipped period, are expected. Heavy flooding, bleeding between periods, or any bleed after twelve months without one is not, and needs a doctor.
Perimenopause is the years in which the ovaries wind down and the menstrual cycle becomes unpredictable before it stops. It is not a single event. The NHS puts the usual age at 45 to 55, but the range is wider and the duration varies more than most people expect. What matters clinically is the pattern of bleeding, not the number on the calendar.
What perimenopause does to the cycle:
The cycle is run by a conversation between the brain and the ovaries. Each month the pituitary sends follicle-stimulating hormone (FSH) to the ovary, the ovary grows a follicle, produces oestrogen, releases an egg, and then produces progesterone from the corpus luteum for the second half of the cycle. Progesterone is what makes the lining shed in an orderly way.
As the supply of follicles falls, that conversation becomes unreliable. Inhibin B, a hormone the follicles make, drops first, so FSH rises. Cycles may shorten as the ovary responds faster, then lengthen as ovulation becomes hit and miss. In many perimenopausal cycles no egg is released at all. Without ovulation there is no corpus luteum, and without a corpus luteum there is no progesterone. The lining is then exposed to oestrogen without the progesterone that normally limits its growth, so it builds up and sheds unpredictably. That is why the same person can have a light period, then a period two weeks later, then nothing for ten weeks, then a very heavy bleed.
NICE guideline NG23 makes this pattern the diagnostic test. In an otherwise healthy woman aged 45 or over, perimenopause is identified without laboratory tests when recently started vasomotor symptoms occur alongside any change in the menstrual cycle. Menopause is identified when there has been no period for at least 12 months and the person is not using hormonal contraception. No blood test is required for either.
A changing cycle, stage by stage:
The staging system used in menopause research is STRAW+10, updated in 2012. It describes bleeding criteria rather than symptoms, which makes it a useful map when your cycle stops behaving.
| Stage | Bleeding criterion | What it can look like |
|---|---|---|
| Late reproductive | Cycles still regular, often shorter | Periods arrive a day or two earlier than they used to |
| Early transition | Consecutive cycles differ by seven days or more, recurring within ten cycles | A 26-day cycle followed by a 34-day cycle, then back again |
| Late transition | No bleeding for 60 days or longer | A missed period, sometimes followed by a heavy one |
| Postmenopause | 12 months without a period | No bleeding; any bleed at this point is abnormal |
Two details in that table are easy to miss. The seven-day difference has to be persistent, meaning it recurs within about ten cycles, rather than being a one-off bad month. And the 60-day gap in the late transition counts from the first day of the last period, not from when you notice it has been a while.
Duration is genuinely variable. An analysis of the SWAN cohort published in 2017 followed women with daily menstrual calendars and found that the median length of the transition ranged from roughly 4.4 years in those who started at the oldest ages to roughly 8.6 years in those who started youngest. Earlier onset meant a longer transition, and smoking was associated with both an earlier start and a shorter one.
Irregular periods in perimenopause: what is normal:
The NHS describes a typical cycle as somewhere between 21 and 35 days. That range is a population average, and perimenopause deliberately widens it. These patterns are common and do not, on their own, need investigation:
- Cycles that vary by a week or more from one month to the next.
- A missed period, followed by a period that arrives more or less on time.
- Periods that are heavier or lighter than you are used to.
- Period pain that changes in severity, or cramps that start earlier.
- Bleeding that lasts a day or two longer than usual.
- Hot flushes, night sweats, disturbed sleep, or mood changes alongside the cycle changes.
The useful question is not whether your periods are regular. It is whether the pattern has changed from your own baseline and whether anything else is happening at the same time. A woman whose cycle has always been 32 days and is now sometimes 24 is showing the same process as a woman whose cycle was 28 days and is now 38.
Irregular periods in perimenopause: red flags:
Some bleeding patterns are not explained by the transition and should be assessed. The distinction is not between light and heavy. It is between bleeding that fits the expected variability and bleeding that suggests a structural cause, a hormonal cause outside the menopause, or, rarely, cancer.
Any vaginal bleeding after 12 months without a period is postmenopausal bleeding. NICE guideline NG12 recommends a suspected cancer pathway referral for endometrial cancer in anyone aged 55 or over with unexplained postmenopausal bleeding that cannot be attributed to HRT, and recommends considering the same referral for those under 55. This is not a wait-and-see symptom.
The NHS lists these as reasons to see a GP: bleeding between periods, bleeding during or after sex, periods lasting longer than seven days, needing to change a pad or tampon every one to two hours, needing to use two products at once, clots larger than about 2.5cm, and bleeding that soaks through clothes or bedding. The NHS womb cancer page adds a change in vaginal discharge, pain in the lower back or pelvis, and pain during sex.
| Bleeding pattern | What it may mean | What to do |
|---|---|---|
| Cycle length varies by a week or more | Expected early transition | Track it; discuss at a routine appointment |
| A single missed period, then normal bleeding | Expected late transition if you are over 45 | Track it; check pregnancy if there is any chance |
| Bleeding between periods or after sex | Polyp, fibroid, cervical or endometrial cause | Book a GP appointment promptly |
| Periods over seven days, or flooding every one to two hours | Heavy menstrual bleeding, possible anaemia | See a GP within days |
| Any bleeding after 12 months without a period | Postmenopausal bleeding | Same-week GP appointment; NICE NG12 referral criteria apply |
| Unscheduled bleeding beyond six months of starting HRT, or three months after a change | Needs assessment on HRT | Contact the prescriber promptly |
Why the pattern changes, and what it is not:
Anovulatory cycles explain most of the variation, but they are not the only cause. Fibroids, adenomyosis, endometrial polyps and thyroid disease all become more common in the same decades, and each can produce heavy or irregular bleeding that has nothing to do with falling oestrogen. Significant weight change, severe stress and some contraceptives alter bleeding too. Rarely, heavy or irregular bleeding is the first sign of endometrial cancer, which is why persistent intermenstrual bleeding is investigated rather than attributed to hormones.
It is also worth being clear about what perimenopause is not. It is not diagnosed by a low oestradiol level, and it is not excluded by a normal one. Hormone levels swing widely from week to week in the transition, so a single result says very little about the stage you are in.
What tests are useful:
NICE NG23 advises against using anti-Müllerian hormone, inhibin A or B, oestradiol, antral follicle count or ovarian volume to identify perimenopause or menopause in women aged 45 or over. It also advises against using FSH to identify menopause in anyone using combined oestrogen and progestogen contraception or high-dose progestogen, because those treatments suppress the result. FSH is considered only to confirm menopause in people aged 40 to 45 with symptoms that include a change in the menstrual cycle, or in people under 40 where premature ovarian insufficiency is suspected.
For heavy bleeding, NICE NG88 recommends a full blood count for every woman, in parallel with treatment, because chronic blood loss causes iron deficiency anaemia. It advises against testing female hormones in heavy menstrual bleeding and against testing thyroid function unless there are other signs of thyroid disease.
Investigation of the uterus depends on the history. An ultrasound scan and outpatient hysteroscopy are both first-line options, and the choice depends on whether the history suggests polyps, submucosal fibroids or endometrial pathology. NICE NG88 recommends considering an endometrial biopsy at hysteroscopy for women at higher risk of endometrial pathology, including those with persistent intermenstrual or persistent irregular bleeding.
Treatment if bleeding is heavy or unpredictable:
Treatment depends on what is causing the bleeding, whether contraception is wanted, and whether fertility matters. NICE NG88 recommends considering the levonorgestrel-releasing intrauterine system (LNG-IUS) as the first treatment for heavy menstrual bleeding when no pathology is found, when fibroids are under 3cm, or when adenomyosis is suspected or diagnosed. It advises explaining that the bleeding pattern may change for the first few cycles and that it can take at least six cycles to see the full benefit.
| Option | What it does | Practical notes |
|---|---|---|
| LNG-IUS | Thins the lining; usually makes periods much lighter | First treatment per NICE NG88 where no cause is found; also contraception |
| Tranexamic acid | Reduces bleeding during the period | Taken only on bleeding days |
| NSAIDs | Reduce bleeding and period pain | Includes mefenamic acid and naproxen |
| Combined hormonal contraception | Regulates the cycle and reduces flow | Also contraception; not suitable for everyone |
| Cyclical oral progestogen | Produces a predictable withdraw bleed | Often used when periods are irregular |
| Referral for hysteroscopy | Finds and treats polyps or fibroids | Offered when treatment fails or risk factors exist |
For perimenopausal irregular bleeding without heavy loss, the combined pill or a progestogen can make the cycle predictable, and the combined pill also provides contraception. Perimenopause is not infertility: ovulation still happens in some cycles, and pregnancy remains possible until menopause is confirmed. That is another reason a missed period in your 40s is worth checking rather than assuming.
Bleeding on HRT is a separate question:
Once you start systemic HRT, the rules change. NICE NG23 explains that vaginal bleeding is a common side effect during the first six months of systemic HRT, and within any three months of changing the dose or preparation. It advises seeking medical help promptly for unscheduled bleeding beyond those windows. The guideline also notes that evidence on unscheduled bleeding during HRT is limited, and points clinicians to the British Menopause Society guidance on the subject.
In practice, that means breakthrough bleeding soon after starting or adjusting HRT is usually expected, while bleeding that starts later, becomes heavier, or continues after the expected window is assessed. Do not assume it is the dose settling.
Tracking that helps your GP:
A short written record is more useful than memory. Note the first day of each period, how many days the bleeding lasts, how heavy it is compared with your normal, any clots, and any bleeding between periods or after sex. Note the dates you started or changed HRT or contraception, because the expected bleeding windows depend on them. A phone app or a paper diary both work. Three months of dates turns a vague story into a pattern a clinician can act on.
When to see a doctor:
- Any vaginal bleeding after 12 months without a period, even a small amount or a single spot.
- Bleeding between periods, after sex, or a discharge that has changed.
- Periods lasting more than seven days, or soaking a pad or tampon every one to two hours.
- Clots larger than about 2.5cm, or bleeding that soaks through clothes or bedding.
- Unscheduled bleeding on HRT after the first six months, or more than three months after a change in dose or preparation.
- Pelvic pain, pain during sex, or a new feeling of pressure or swelling in the lower abdomen.
- Periods that have stopped because of contraception or treatment, followed by new bleeding.
- Dizziness, breathlessness or unusual tiredness with heavy periods, which may mean anaemia.