Why You Bloat, Retain Water and Sometimes Get Diarrhoea Before Your Period

By Tina Chagoury, Clinical Dietitian · 5 min read · 2026-10-05

These symptoms seem unrelated. They are the same mechanism, and it is entirely predictable once you understand which hormones do what.

Why do you feel bloated, retain water, and on some days experience diarrhoea before your period? These seem like separate problems. They are the same mechanism.

Fluid balance and gut motility are both directly and heavily influenced by hormones.

What happens across the cycle

In the second half of the cycle, after ovulation, progesterone rises.

Progesterone slows gut motility. Food moves through the digestive tract more slowly. That produces the bloating, the fullness and the constipation that so many women experience in the week or so before their period.

Meanwhile, oestrogen and progesterone together affect how the body handles sodium and water, which is where the fluid retention comes from. The ring that will not fit, the tighter clothes, the puffiness, the two kilograms on the scale that appear and disappear.

Just before bleeding begins, progesterone drops sharply.

That drop is what flips the pattern. Gut motility speeds up again, sometimes considerably, and compounds released around the onset of menstruation stimulate the bowel further. Which is why some women go from constipation to loose stools within a day or two.

It is not two conditions. It is one hormonal sequence with two phases.

Why it is worse for some women than others

The severity depends on what else is going on:

• Blood sugar instability makes premenstrual symptoms markedly worse, including bloating and fluid retention
• Existing gut inflammation amplifies everything
• A high inflammatory load makes blood vessels leakier, which increases retention
• Stress and cortisol, because cortisol affects aldosterone, the hormone governing water balance
• Low potassium relative to sodium
• Dehydration, which causes the body to hold water more tightly as a protective response

In perimenopause the whole pattern becomes less predictable, because progesterone is declining and oestrogen is fluctuating rather than following a clean rhythm.

What actually reduces it

The interventions are not premenstrual. They are what you do all month.

• Stabilise blood sugar. Protein at every meal, three structured meals, no all day grazing. This alone changes premenstrual symptoms substantially for most women.
• Reduce the inflammatory load. Processed food, refined and fried oils, processed sugar.
• Fibre and hydration, which support motility directly.
• Move. The lymphatic system has no pump of its own. Walking and resistance training drive drainage.
• Balance sodium with potassium, which vegetables do for you.
• Magnesium, which many women find helpful for both bowel function and premenstrual symptoms.

When it is not normal

Premenstrual bloating is common. Bloating every single day is not.

When I ask a woman how long she has had bloating and she says she always has it, that is a different problem and it deserves investigation rather than being folded into the cycle.

The same applies to periods heavy enough to interfere with your life, or painful enough to stop it. Very common. Not normal. Worth investigating properly rather than living with.

This article is nutrition education, not medical advice. Persistent digestive symptoms or very heavy periods should be assessed by a doctor.

Tina Chagoury is a licensed clinical dietitian specialising in women's hormonal health and gut health, and the creator of the Hormone Reset Method™. Learn more and start here.