Perimenopause is the stretch before your last period, when hormone levels start fluctuating rather than simply declining. It is not the same thing as menopause, which is a single point in time — the day twelve months after your final period. Everything before that is perimenopause, and it can last years.
That distinction matters, because a lot of women spend those years being told they are "too young" for this.
When it starts, and how long it lasts
The NHS puts the average age of menopause in the UK at 51. Perimenopause commonly begins in the mid-forties, but starting in the late thirties is well within normal range. The NHS notes perimenopausal symptoms can begin months or years before periods stop.
Length varies enormously. For some it is a year or two. For others it runs the better part of a decade. There is no test that tells you where you are in it — blood tests for follicle-stimulating hormone are unreliable during perimenopause precisely because the hormones are fluctuating, which is why NICE advises diagnosing on symptoms alone in women over 45.
The symptoms nobody mentions
Hot flushes and night sweats are the ones everyone knows. They are also not the ones most women find hardest.
- Brain fog — losing words mid-sentence, rereading the same paragraph, walking into rooms without knowing why
- Sleep disruption — particularly waking in the small hours and struggling to get back down
- Mood changes — irritability, anxiety, or a flatness that does not feel like ordinary low mood
- Joint aches, which surprise people because they seem unrelated
- Changes to periods — closer together, further apart, heavier, lighter, unpredictable
A 2025 cohort study in Menopause looking at cognitive and mood-related symptoms across a large group of perimenopausal and menopausal women found these complaints are not fringe experiences — they are among the most commonly reported, and they cluster together.
Why brain fog happens
Oestrogen is not only a reproductive hormone. It has receptors throughout the brain, including in the hippocampus and prefrontal cortex — regions doing memory and executive function. Oestrogen also modulates dopamine and serotonin signalling.
So when oestrogen starts fluctuating unpredictably, the effects are not confined to the reproductive system. The cognitive symptoms are not imagination, and they are not early dementia. For most women they ease as hormones settle after menopause.
What the UK guidance actually recommends
NICE guideline NG23 is the reference point for UK clinicians. In short:
- HRT is the first-line treatment for vasomotor symptoms such as hot flushes and night sweats, and NICE is clear that for most women under 60 the benefits outweigh the risks
- CBT is recommended for low mood and anxiety arising from menopause
- NICE explicitly does not recommend antidepressants as a first-line treatment for low mood arising from menopause in women who have not been diagnosed with depression
- Diagnosis in women over 45 should be based on symptoms, without blood tests
That last point is worth taking to an appointment. Being told you need a blood test to confirm perimenopause, when you are over 45 and symptomatic, is not what the guideline says.
When to see your GP
See a GP if symptoms are affecting your daily life, if you are under 45 and your periods have become irregular or stopped, or if you are unsure whether what you are experiencing is perimenopause at all. If you are under 40, the NHS advises seeking assessment for premature ovarian insufficiency, which is managed differently.
It is also worth knowing that you can ask to see a different clinician. Menopause care in UK general practice is uneven, and a doctor with a particular interest in it will often get you further faster.
Sources
- NHS. Menopause and perimenopause. nhs.uk — symptoms, timeline, and when to seek help.
- National Institute for Health and Care Excellence. Menopause: identification and management. NICE guideline NG23.
- Prevalence of Cognitive and Mood-Related Symptoms in a Large Cohort of Perimenopausal and Menopausal Women. PubMed Central, PMC11738833.