Your menstrual cycle begins on the first day of your menstruation and continues up to, but not including, the first day of your next period. How long the period lasts and how often it comes is different for everyone. Women’s cycles range from 21 to 40 days or more, with an average of around 28 days.

The length of your cycle may change from month to month. Periods usually last between 2 to 7 days and blood loss varies from light to heavy and bright red to dark brown.

Any alteration in what you consider to be your normal cycle can be a cause for concern. Variations may mean you have a period more often or less often than is normal for you or you may not have a period at all. You may also bleed more or less than normal, and the length of your period may also change.

The absence of periods (or amenorrhoea) usually occurs because an egg is not produced in the ovaries. The most common causes of absent periods are:

• not having started puberty (if your periods don’t start by the time you’re 16, see a doctor)
• pregnancy
• breastfeeding
• menopause

There are several other factors that may cause your periods to stop or to become lighter or less frequent, such as:

• excessive exercise
• being underweight or excessive dieting
• feeling upset or stressed
• severe long term illness
• a hormonal imbalance
• a condition such as polycystic ovarian syndrome
• a thyroid disorder or pituitary disorder
• stopping or starting the contraceptive pill or other hormonal contraception (including the patch, IUS (Mirena) coil, implant (rods) or injection)
• some medicines
• disorders of the womb or ovaries

Experiencing changes in the frequency or duration of periods is common in most women at some point in their lives. Any treatment or investigation of irregular or light periods will depend on the likely cause. There may, for example, be no need for treatment if you are nearing the menopause as irregular periods are common during this time.