Brain Fog in Perimenopause: What It Is and Why It Happens

You are halfway through a sentence and the word is gone, you stand in a room and forget why you walked in, you reread the same paragraph three times and none of it lands. You have always been sharp, and now you are asking yourself whether something is wrong with you.

I am a Former Registered Nurse, and I did not recognise my own perimenopause when it arrived, because the symptom I noticed first was not a period change. It was the word gone mid-sentence, the sense of static where clear thinking used to be.

Brain fog affects 40 to 50 percent of women in perimenopause, a figure noted in a large 2026 study from King's College London and Imperial College London that also confirms what women have been saying for years, that for most women it settles.^1,2

What brain fog actually is

Brain fog is not a medical diagnosis, it is the name given to a cluster of experiences: forgetfulness, mental slowing, word-finding trouble, and difficulty holding focus.¹ A large 2026 study from King's College London and Imperial College London followed 14,234 women aged 45 to 55 and found that women in perimenopause and postmenopause had significantly higher odds of reporting poor memory and brain fog than women who were still premenopausal.¹

Why this happens, the oestrogen connection

Oestrogen is not only a reproductive hormone, it acts throughout the brain, particularly in the hippocampus, the amygdala and the prefrontal cortex, the regions that govern memory, emotional regulation and executive function.^3,4 As oestrogen rises and falls through perimenopause, far more than in other stages of life, these regions of the brain are affected.

Brain imaging research by Dr Lisa Mosconi has tracked this in real time, using MRI and PET scans across the menopause transition. This research found measurable dips in grey matter volume, brain connectivity and brain energy metabolism, and importantly, found that grey matter and brain energy levels partially recover after menopause, tracking with preserved cognition.⁵

Many women notice that low mood, anxiety and mental fog seem to show up together rather than as separate problems, and there is a plausible biological reason for that. Human brain-imaging research has shown that giving oestrogen to postmenopausal women increases the availability of a key serotonin receptor across wide regions of the brain.⁶ This receptor is one of the ways your brain uses serotonin to regulate mood, so when more of it is available, your brain has more capacity to use the serotonin it already has. The reverse has not been directly tested in the same way, but the logical read is that if oestrogen raises receptor availability, falling oestrogen likely lowers it, leaving your brain with less capacity to use serotonin. This is a reasonable inference from the evidence, not yet a directly proven finding, but it helps explain why these three symptoms so often arrive as a package rather than on their own.

Researchers at King's College London and Imperial College London tested 14,234 women on eight validated cognitive tasks, expecting perimenopausal women to perform slightly worse than other groups. That is not what they found. Objective performance differences were tiny across premenopause, perimenopause and postmenopause. This research did not dismiss the subjective experience of brain fog, but it found no evidence of measurable cognitive decline behind it.¹

Why it feels so much bigger

The King's College London and Imperial College London study measured both what women reported and how they actually performed on eight validated cognitive tasks.¹ The self-reported symptoms and the objective test scores were only weakly related. What was strongly related to brain fog, consistently across every group in the study, was psychological symptoms: anxiety, low mood and mood swings.¹

This does not mean brain fog is "just anxiety" or imagined, it means brain fog is multi-factorial, very likely reflecting short, transient lapses that do not show up on a 20-minute cognitive test.¹ The researchers themselves reiterate that patient-reported experience needs to be taken seriously and not be dismissed because a test score looks normal.¹ This means the tools available right now are better at measuring global cognitive ability than at capturing the brief, disruptive lapses you are living with day to day, and it does not mean your symptoms are not impacting you.

Brain fog is not early dementia

This is very often the quiet fear sitting underneath the practical frustration. Australian researchers at Monash University and the University of Melbourne found that more than two-thirds of women report cognitive symptoms during the menopause transition, and that these symptoms are not linked to higher dementia risk and commonly settle after the transition.²

The Australasian Menopause Society frames brain fog the same way: common, real, and for most women, temporary.⁷ There is a meaningful difference between ordinary perimenopausal fog and something that needs medical review. Typical menopausal fog fluctuates, tends to involve word-finding and misplaced items, and still allows you to function day to day. Signs that deserve a GP conversation include getting lost in familiar places, real difficulty with previously familiar tasks, other people noticing changes more than you do, rapid worsening, or confusion about time and place.⁷

If any of that second list sounds like you, please raise it with your GP, not because it is likely to be serious, but for ruling out other causes: thyroid disease, sleep apnoea, depression, iron deficiency, vitamin B12 deficiency, or heavy alcohol use, as these are commonly missed contributors to cognitive symptoms in midlife.⁸

What actually helps

There is no single fix, because brain fog is not a single problem.

Sleep is closely tied to cognitive symptoms. Sleep difficulties were reported more frequently by postmenopausal women, and psychological symptoms closely linked to sleep were the strongest correlate of brain fog across every group studied.¹

Regular movement, a nutrient-dense diet, reducing alcohol, staying mentally and socially engaged, and treating anxiety or low mood directly are all supported by current guidance as part of managing menopausal brain fog.⁷

If your symptoms are significantly affecting your quality of life, this is worth raising with your GP as part of a broader conversation about your perimenopause, alongside the lifestyle steps above.

What this means for you

Your brain is moving through a real neuroendocrine transition, and the fog has both a measurable biological basis and a strong tendency to lift as you move through to the other side of it.

Three takeaways

1. Brain fog in perimenopause is a recognised, common, and biologically real symptom, it is not early dementia for the vast majority of women.

2. Your subjective experience of fog matters even when a cognitive test looks normal, because current tests are not well suited to capturing brief daily lapses.

3. If brain fog is accompanied by getting lost in familiar places, a rapid change others have noticed, or real difficulty with previously familiar tasks, raise it with your GP so other causes can be ruled out.

This is not your body failing you. It is your body changing, and asking for a different kind of support.

Start by noticing patterns. Write down when the fog is worse, how your sleep was the night before, and what else was going on. Patterns are information, and information is something you can act on.

Download the free Menopause Symptom Checklist at menopausehub.com.au/mht-checklist.

This information is educational only and does not constitute personal medical advice. Always consult a qualified health professional about your individual circumstances. © Menopause Hub 2026.

REFERENCES

1. Naysmith LF, Ward H, Elliott P, et al. Cognition and the menopause transition: cross-sectional evidence from a large community cohort. npj Women's Health. 2026;4:14.

2. Gurvich C, Hickey M, Spector A, et al. Advances in understanding of cognitive symptoms during menopause. The Lancet Obstetrics, Gynaecology & Women's Health. 2026 Apr.

3. Maki PM, Jaff NG. Brain fog in menopause: a health-care professional's guide for decision-making and counseling on cognition. Climacteric. 2022;25(6):570 to 578.

4. Russell JK, Jones CK, Newhouse PA. The role of estrogen in brain and cognitive aging. Neurotherapeutics. 2019;16:649 to 665.

5. Mosconi L, Berti V, Dyke J, et al. Menopause impacts human brain structure, connectivity, energy metabolism, and amyloid-beta deposition. Scientific Reports. 2021;11:10867.

6. Moses-Kolko EL, Berga SL, Greer PJ, Smith G, Cidis Meltzer C, Drevets WC. Widespread increases of cortical serotonin type 2A receptor availability after hormone therapy in euthymic postmenopausal women. Fertility and Sterility. 2003;80(3):554 to 559.

7. Australasian Menopause Society. Brain fog and memory/cognition and menopause. menopause.org.au.

8. MHT_Podcast_Extract_10_BrainHealth_MultipleAuthors, synthesis of McKay, Newson, Maki and Streicher podcast material, Menopause Hub Research Library, 2026.