It is normal for your periods to stop at certain points, although absent periods are sometimes the result of an underlying health problem.

In some cases, a cause may not be found.

Naturally absent periods

It is normal for periods to stop while you are pregnant or breastfeeding, and they often become less frequent during the menopause (when the ovaries stop regularly producing eggs, usually at around 50 years of age).

Becoming pregnant without realising is a surprisingly common cause of an absent period. This often happens when your method of contraception fails without your knowledge.

In the case of pregnancy and breastfeeding, your periods will eventually return. After the menopause, you will stop having periods altogether.

Developmental delays

Girls often start having periods from around 12 years of age. However, some girls don't have their first period until later, particularly if this was the case with their mother or older sisters.

This is usually nothing to worry about, as most of these girls will eventually start having periods by the time they are 16-18 years old.

Contraception

Some women who use a contraceptive implant (such as a Mirena coil), a contraceptive injection or, less commonly, the contraceptive pill (sometimes called the 'mini pill') may find their periods become irregular or stop completely.

Your periods should start again once you stop using these forms of contraception, although occasionally these effects can persist.

If you have not been using these types of contraception for six months or more and still have not had your period, contact your GP for advice.

Medical conditions

There are also medical conditions that can cause absent periods. Depending on when these conditions develop, they can mean a girl doesn't start having periods by the expected age (primary amenorrhoea), or a girl or women who has previously had periods stops having them (secondary amenorrhoea).

Some of the main medical conditions that can cause absent periods are described below.

Polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a common condition responsible for as many as one in three cases of absent periods.

The features of PCOS include:

• a number of under-developed egg sacs (follicles) in your ovaries
• the ovaries do not regularly release eggs (ovulate)
• having high levels of "male hormones" (androgens) in your body

As well as causing absent periods, other symptoms of PCOS include excessive body hair, problems getting pregnant and weight gain.

Hypothalamic amenorrhoea

The menstrual cycle is regulated by part of the brain known as the hypothalamus. This produces hormones that cause the ovaries to release eggs. In cases of hypothalamic amenorrhoea, the hypothalamus stops producing these hormones and the menstrual cycle stops.

Exactly why the hypothalamus does this is unclear, but it has been linked to:

• excessive weight loss, for example, due an eating disorder such as anorexia nervosa
• excessive exercise
• stress
• long-term (chronic) illnesses - such as heart disease and uncontrolled diabetes

Hypothalamic amenorrhoea is more common in women whose profession requires a combination of physical fitness and maintaining a low body weight, such as athletes and dancers.

Even if you are not concerned about not having a period, you should still seek medical advice if you think you have hypothalamic amenorrhoea, as it can lead to brittle bones (osteoporosis) and put you at risk of a bone fracture.

Hyperprolactinaemia

Hyperprolactinaemia means you have excessively high levels of a hormone called prolactin in your body.

High levels of prolactin are normally only required after you have given birth, as they help stimulate production of breast milk. Having high levels at other times can disrupt the normal menstrual cycle and lead to absent periods.

Hyperprolactinaemia is thought to affect around 1 in every 200 women and can have a wide range of causes, such as:

• a brain tumour 
• a severe head injury
• kidney disease 
• cirrhosis (scarring of the liver)

Hyperprolactinaemia can also arise as a side effect of treatments and medications, such as:

• radiotherapy 
• antidepressants 
• calcium channel blockers (a medication used to treat high blood pressure)
• omeprazole (a medication used to treat stomach ulcers)

Women who regularly use heroin also often develop hyperprolactinaemia.

Premature ovarian failure

Premature ovarian failure is when the ovaries stop producing eggs in women who should still be young enough to ovulate (usually 45 or younger).

It is estimated that premature ovarian failure affects 1 in every 100 women before the age of 40 and 1 in every 20 women before the age of 45.

It is thought many cases of premature ovarian failure are caused by the immune system malfunctioning and attacking the ovaries. The condition has also been linked to having chemotherapy or radiotherapy.

You should seek medical advice if you suspect you have premature ovarian failure, even if you are not concerned about having periods. This is because having this condition can put you at risk of osteoporosis and heart disease.

Thyroid conditions

The thyroid gland is found in the neck. It produces hormones that are released into the bloodstream to control the body's growth and metabolism. They affect processes such as heart rate and body temperature, and help convert food into energy to keep the body going.

In some women, the thyroid gland can:

• produce too much thyroid hormone – this is known as having an overactive thyroid gland or hyperthyroidism 
• produce too little thyroid hormone – this is known as having an underactive thyroid gland or hypothyroidism

Both hyperthyroidism and hypothyroidism can cause absent periods.

Genetic conditions

Although uncommon, absent periods can also be caused by a number of conditions caused by problems in your genes. These include:

• Turner syndrome – a condition affecting around 1 in every 2,000 girls, which causes those affected to be born with ovaries that do not produce the hormones required to trigger the menstrual cycle
• Kallmann syndrome – a rare condition affecting around 1 in every 10,000 births, where hormones that normally trigger sexual development are missing
• androgen insensitivity syndrome – a rare condition affecting around 1 in every 20,000 births, where a child is genetically male but their genitals can appear to be female

Birth defects

In rare cases, absent periods may be caused by a problem in the development of a girl's reproductive system that has been present from birth, such as having no womb or vagina.