Why do bowel habits change during a period?
Why bowel movements speed up during a period, what prostaglandins have to do with it, why it varies from woman to woman, and when to see a specialist.
Faster bowel movements, looser stools or mild diarrhoea during a period are common. The reason is that prostaglandins, the substances the womb produces to bring on menstrual bleeding, affect the bowel as well. What follows explains why, why it is not equally strong in everyone, and when it is worth seeing a specialist.
What causes it?
During a period the womb produces hormone-like substances called prostaglandins so that it can shed its lining. These substances contract the muscle of the womb and bring about the menstrual bleeding.
But they do not stay in the womb. They pass into the bloodstream and can affect smooth muscle elsewhere in the body. Because the muscle in the bowel wall is sensitive to them, bowel movements speed up, and stools are passed before there is time for enough water to be absorbed in the bowel — which produces looser stools or diarrhoea.
Why is it not equally strong in everyone?
- Individual prostaglandin levels: women with severe period pain generally produce higher amounts of prostaglandins, which can make the bowel symptoms more pronounced.
- A sensitive bowel: in people whose bowel is more sensitive than usual — those with irritable bowel syndrome, a condition of heightened bowel sensitivity with pain and irregular bowel habit — even normal levels of prostaglandins can provoke a stronger reaction.
When should you see a specialist?
Mild to moderate changes in bowel habit that begin with the menstrual bleeding and pass when it ends are generally regarded as normal. In the following situations, though, it matters to see a specialist:
- blood in the stool
- diarrhoea that continues throughout the month rather than only during the period, or that wakes you at night
- a high fever alongside it
- unexplained weight loss
- severe abdominal or pelvic pain that painkillers do not relieve
These signs can point to another underlying condition — endometriosis, for example, where tissue resembling the lining of the womb settles outside the womb and can affect the bowel too — and call for a more detailed assessment.
Frequently asked questions
Is it normal for this to happen every month? If it starts with the period, ends with the bleeding and is not severe, it is regarded as normal.
Is this related to endometriosis? In some cases, yes. Where severe pain, deep pain on opening the bowels, or blood in the stool accompany it, that possibility should be assessed.
This page is for information. It does not replace an examination or individual medical advice.