Your menstrual cycle can be disturbed if you change your method of contraception or you have an imbalance of the hormones oestrogen and progesterone.

It's not unusual to have a hormone imbalance for a few years after puberty and before the menopause. This can cause your menstrual cycle to become longer or shorter. Your periods may also become lighter or heavier.

If your irregular periods are caused by these age-related factors, you won't usually need to see your GP.

Lifestyle

The following lifestyle factors can also upset your balance of hormones and cause irregular bleeding:

• extreme weight loss or weight gain
• excessive exercise
• stress 

Contraceptives

An intrauterine system (IUS) or contraceptive pill may cause spotting between periods.

An intrauterine device (IUD) doesn't cause irregular periods, but can cause heavy bleeding or painful bleeding.

Small bleeds, known as breakthrough bleeds, are common when the contraceptive pill is first used. They're usually lighter and shorter than normal periods, and usually stop within the first few months.

Polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) occurs when very small cysts (small, fluid-filled sacs) develop in the ovaries.

The usual symptoms of PCOS are irregular or light periods or no periods at all. This is because, in women with PCOS, ovulation (the release of an egg) may not take place as often as normal.

The production of hormones may also be unbalanced, and you could have higher levels of testosterone than normal. Testosterone is a male hormone that women usually have a small amount of.

Gynaecological problems

Irregular bleeding can also be caused by an unsuspected pregnancy, early miscarriage, or problems with the womb or ovaries. Your GP may refer you to a gynaecologist (a specialist in conditions of the female reproductive system) if further investigation and treatment is needed.

Thyroid disorders

A thyroid disorder is another possible, but rare, cause of irregular periods. The thyroid gland, found in the neck, produces hormones that maintain the body's metabolism (the chemical processes your body uses to turn food into energy). Your GP may test for a thyroid problem by taking a blood test to check levels of thyroid hormones in your blood.

Read more about an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism).