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Page last reviewed: 13/07/2011

A period is when blood leaves the womb through the vagina. It happens approximately every 28 days, although anywhere between 24 and 35 days is common. A period is part of a woman's menstrual cycle.

It is possible for your periods to begin between the ages of eight and 16 years, but they most commonly start around the age of 12. Your periods will continue until the menopause,which usually occurs around 45 to 55.


As a baby girl, you are born with all your eggs stored in follicles (sacs) in your ovaries. As puberty begins, your body starts to produce hormones such as oestrogen and progesterone, which cause your eggs to mature. This is the beginning of your first menstrual cycle.

Your menstrual cycle begins on the first day of bleeding, and it continues up to, but not including, the first day of your next period.

The menstrual cycle explained

During the menstrual cycle, hormones released by the pituitary gland in your brain stimulate your ovaries. As a result, some of the eggs stored in the follicles of your ovaries begin to grow and mature.

These follicles start to produce the hormone oestrogen and, as a result, the oestrogen level in your bloodstream rises. This causes your womb lining to thicken as it prepares to receive a fertilised egg.

If you have recently had sex (within several days of the egg being released) and there are sperm in your fallopian tube, the egg may become fertilised.

However, if there are no sperm to fertilise the egg, your oestrogen and progesterone levels will decrease and the lining of the womb will start to break down. This marks the start of your period.

Body changes

Different amounts of each of the female hormones are produced by your body at different times during your cycle. As a result, lots of body changes occur throughout your menstrual cycle.

For example, you may develop swollen breasts and/or have mood changes. Your basal body temperature (your temperature first thing in the morning) will also change depending on which point you are at in your cycle.

Page last reviewed: 13/07/2011

During your period, you will experience bleeding that can last for up to eight days, although it usually lasts about five days. The bleeding is heaviest in the first two days.

Your flow may seem heavy, but the amount of blood lost is normally only enough to fill between five and 12 teaspoons.

The cycle of your hormones may affect you physically and emotionally. However, some women have few or no symptoms during their period. 

For information on period problems, such as irregular, painful or heavy periods, see Complications.

Premenstrual syndrome

Premenstrual syndrome (PMS) is the name given to the set of physical, psychological and emotional symptoms that appear in the days leading up to a woman's monthly period. It is also known as premenstrual tension (PMT).

Symptoms include:

  • feeling irritable and bad-tempered
  • fluid retention and feeling bloated
  • mood swings

Usually, these improve when your period starts, and they disappear a few days afterwards. For more information, see Health A-Z :pre menstrual syndrome

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Page last reviewed: 13/07/2011

Sometimes you may experience problems with your periods. For example, you may get:

  • heavy periods (menorrhagia)
  • painful periods (dysmenorrhoea)
  • irregular periods
  • you may not have any periods at all (amenorrhoea)

Heavy periods (menorrhagia)

The amount of blood lost during a period varies from woman to woman. As a result, it is very hard to diagnose menorrhagia.

If you feel that your periods are so heavy they are disrupting your everyday life and making you feel miserable, you should visit your GP. For example, you may have heavy periods if:

  • you are using excessive amounts of tampons or sanitary towels
  • blood leaks through your clothes
  • you need to use a sanitary towel and a tampon to prevent leakage

For more information, see Health A-Z: heavy periods

Painful periods (dysmenorrhoea)

Painful periods, also known as dysmenorrhoea, means feeling pain in your abdomen (lower stomach), pelvis, lower back, thighs and/or vagina shortly before and during your period.

Painkillers such as non-steroidal anti-inflammatory drugs (NSAIDs) can be used to treat painful periods.

For more information, see  Health A-Z: painful periods.

Irregular periods

Periods can last between two and eight days. If you have irregular periods, the gaps between them will vary, as well as the amount of blood lost and the duration of the bleeding.

There are many possible causes of irregular periods, and treatment will depend on the underlying cause. Possible treatments include changing your method of contraception, relaxation classes and counselling.

For more information, see Health A-Z: irregular periods

Absent periods (amenorrhoea) 

Sometimes, a woman may stop having periods altogether. Usually, this means that no eggs are produced.

There are many possible causes of amenorrhoea, which include severe stress, extreme weight loss, various medications and polycystic ovary syndrome. Treating the underlying cause often brings your periods back. 

For more information, see  Health A-Z: absent periods.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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