Why Does It Occur?

The cause of PMS is not known, but it is thought that the symptoms are triggered by the action of the female sex hormone progesterone, in the weeks or days before menstruation begins. The sex hormones do not cause PMS.

Artificially altering circulating progesterone and oestrogen does not cause premenstrual syndrome in previously well women, only those already prone to PMS. Studies suggest that the key events are actually occurring in the brain, not the ovary.

There seems to be an abnormal response in the central nervous system to the normal variation in progesterone during the menstrual cycle. For example, metabolites resulting from the breakdown of progesterone, such as allopregnanolone and pregnenolone, have an effect on psychological pathways in the brain such as those mediated by gamma-aminobutyric acid A (GABA-A) receptors.

Low levels of allopregnanolone can cause the psychological symptom of anxiety. One theory is that the brain cells, known as neurones, in women with PMS, metabolise progesterone to pregnenolone rather than allopregnanolone thus causing a deficiency of allopregnanolone and symptoms of anxiety.

Sugar, caffeine and stress may make PMS worse. Women are also more likely to experience the condition if a close female relative also had a similar problem, suggesting there may be a genetic basis.