Diarrhoea in perimenopause: why bowels become unpredictable
Bowel habit commonly becomes unpredictable in perimenopause, and diarrhoea is as frequently reported as constipation. Two mechanisms dominate: prostaglandins released around a period act on the bowel as well as the uterus, and falling progesterone removes a brake on gut motility. Irritable bowel syndrome also frequently worsens in midlife. Persistent change in bowel habit, blood in the stool, or weight loss must be investigated rather than attributed to hormones.
Start with what must be excluded
A persistent change in bowel habit lasting more than a few weeks is one of the symptoms that prompts investigation for bowel cancer, and the age at which it becomes relevant overlaps with perimenopause.
Features that need assessment rather than observation: blood in the stool, a persistent change in habit lasting more than three weeks, unexplained weight loss, persistent abdominal pain, or a family history of bowel cancer.
Most cases are not sinister. That is exactly why the unusual ones get missed when everything is attributed to hormones.
The period connection
Many women notice bowels loosening in the days before or during a period, and the mechanism is well described.
Prostaglandins released as the womb lining breaks down do not confine themselves to the uterus. They act on smooth muscle in the bowel too, increasing contraction and speeding transit.
In perimenopause, cycles without ovulation build a thicker lining, and heavier shedding means more prostaglandin. So the same change that makes periods heavier and cramps worse also makes bowel symptoms around them more pronounced.
The progesterone brake
Progesterone relaxes smooth muscle, which slows gut transit. This is why constipation is common in pregnancy and in the second half of a normal cycle.
As ovulation becomes unreliable in perimenopause, that braking effect becomes intermittent. Cycles with progesterone slow things; cycles without it do not. The result is alternating rather than consistently loose or consistently constipated bowels, which is one of the most characteristic features.
What else is common at this age
- Irritable bowel syndrome, which frequently worsens or first presents in midlife and has a well-documented hormonal component
- Coeliac disease, which is diagnosed at every age and is substantially underdiagnosed in adults
- Bile acid malabsorption, a common and treatable cause of chronic watery diarrhoea that is frequently missed
- Lactose intolerance, which can emerge in adulthood
- Microscopic colitis, which disproportionately affects women around this age and causes chronic watery diarrhoea with a normal-looking colonoscopy unless biopsies are taken
- Thyroid overactivity, which speeds transit
- Medications, including metformin, some antidepressants and magnesium supplements
When to see someone
Book for a change in bowel habit persisting more than three weeks, and ask for coeliac serology, thyroid function, full blood count and ferritin as a starting point.
Seek prompt assessment for blood in the stool, unexplained weight loss, or a new persistent change in habit over 50. Ask about bile acid malabsorption and microscopic colitis specifically if chronic watery diarrhoea has been investigated and labelled as IBS without those being considered.
Wondering if this is perimenopause?
Two free next steps: check where your symptoms and cycle sit against the clinical criteria, or go straight to clinicians who treat this every day.
Sources
This page is general information, not medical advice, and it does not recommend or rule out any treatment for you personally. If something here contradicts what your clinician tells you, your clinician knows your situation — this page does not.