PMS is a chronic condition experienced by menstruating women which is characterised by distressing physical, behavioural and psychological symptoms that regularly recur during the luteal phase of the  menstrual cycle (from ovulation to the onset of a period) and that disappear or significantly diminish by the end of the period (menstruation).

The menstrual cycle normally occurs monthly throughout a woman’s reproductive years as the lining of the womb (uterus) gradually grows in thickness and is shed at the period (menstruation). The average length of the menstrual cycle (from the first day of bleeding to the first of another) is 28 days but this is only an average and can vary greatly between women. Also, an individual woman may find that the length of her menstrual cycle varies from month to month.