The exact cause of premenstrual syndrome (PMS) isn't fully understood, but a number of things may contribute to the symptoms.
During your menstrual cycle, levels of hormones such as oestrogen and progesterone rise and fall. Hormone changes are thought to be the biggest contributing factor to many of the symptoms of PMS.
The fact that PMS improves during pregnancy and after the menopause, and when hormone levels are stable, supports this theory.
Chemical changes in the brain
It's also been suggested that changes in hormone levels during the menstrual cycle may affect the levels of certain chemicals in your brain, such as serotonin.
Serotonin is known to help regulate your mood and make you feel happier, so a reduction in the level of serotonin caused by changes in hormone levels may explain the mood changes often associated with PMS.
It may also explain why a type of antidepressant medication that increases serotonin levels – known as selective serotonin reuptake inhibitors (SSRIs) – helps some women with PMS.
There are also a number of lifestyle factors that can increase your risk of PMS. These are outlined below.
Weight and exercise
Research has shown you're more likely to have PMS if you're obese (you have a body mass index of more than 30) and if you do little exercise.
Read more about calculating your BMI and health and fitness.
You may find symptoms of PMS get worse as you become more stressed. While it's not a direct cause, stress can aggravate the symptoms of PMS.
Read more about understanding stress.
Eating too much of some foods and too little of others may also contribute to PMS symptoms. For example, too much salty food may make you feel bloated.
Alcohol and caffeinated drinks can disrupt your mood and energy levels. Low levels of vitamins and minerals may also make your PMS symptoms worse.
Read more about maintaining a balanced diet.