Intrauterine system (IUS)

Page last reviewed: 13/07/2011

An IUS is a small, T-shaped contraceptive device that fits inside the womb (uterus) and releases the female hormone progestogen into the body.

An IUS is a long-lasting and reversible method of contraception, but it is not a barrier method. This means that it cannot stop you getting sexually transmitted infections (STIs).

The IUS is similar to the intrauterine device (IUD), but works in a slightly different way. Rather than releasing copper like the IUD, the IUS releases the hormone progestogen, which prevents pregnancy.

The IUS that is available in Ireland is called Mirena and works for up to five years after being fitted. If you are 45 or older when you have the IUS fitted, it can be left until you reach menopause or you no longer need contraception.

Most women can have the IUS fitted, including women who have never been pregnant or who are HIV positive. It must be fitted by a trained doctor or nurse at your GP surgery, family planning clinic or sexual health clinic.

If it is fitted in the first seven days of your menstrual cycle, the IUS protects against pregnancy immediately. If it is fitted at another time, another type of contraception has to be used for the first seven days.

How it works

The IUS releases the hormone progestogen, which is similar to the natural hormone progesterone that is produced in a woman's ovaries. This prevents pregnancy by stopping fertilised eggs from becoming embedded in the womb.

It does this by making the lining of the womb thinner so it is less likely to accept a fertilised egg. It also thickens the mucus from the cervix (opening of the womb), making it harder for sperm to move through it and reach an egg.

In some women, the IUS also stops the ovaries from releasing an egg (ovulation), but most women will continue to ovulate.

How effective is the IUS?

The IUS is over 99% effective in preventing pregnancy. This means that less than one in every 100 women who use the IUS will get pregnant over five years.

The IUS is very effective as a long-lasting method of contraception. You do not need to remember to take or use contraception to prevent pregnancy. However, it does not protect against STIs.

Page last reviewed: 13/07/2011

Most women can use an IUS, including women who have never been pregnant and those who are HIV positive. Your GP or nurse will ask about your medical history to check if an IUS is the most suitable form of contraception for you.

Your family and medical history will determine whether or not you can use an IUS. For example, this method of contraception may be unsuitable if you think you may already be pregnant or if you have:

  • breast cancer (or have had it) in the last five years,
  • cervical cancer,
  • active disease of the liver,
  • unexplained bleeding from your vagina, for example between periods or after sex,
  • arterial disease or history of serious heart disease or stroke,
  • thrombosis (blood clots) in any vein or artery,
  • an untreated sexually transmitted infection (STI) or pelvic infection,
  • migraines with aura (when you get a forewarning of a migraine attack such as blurred vision or pins and needles), or
  • problems with your uterus or cervix.

Like the IUD, IUS can be unsuitable for women who have untreated STIs. A doctor will usually perform an internal examination to make sure you do not have any existing infections.

Using an IUS after giving birth

An IUS is usually fitted four to six weeks after the birth (vaginal or caesarean). You will need to use alternative contraception from three weeks (21 days) after the birth until the IUS is put in. In some cases, an IUS can be fitted within 48 hours of giving birth.

It is safe to use an IUS when you are breastfeeding and it will not affect your milk supply.

Using an IUS after a miscarriage or abortion

An IUS can be fitted by an experienced doctor or nurse straight after an abortion or miscarriage, as long as you were pregnant for less than 24 weeks. If you were pregnant for more than 24 weeks, you may have to wait a few weeks before having an IUS fitted.


The uterus (or womb) is a hollow, pear-shaped organ in a woman where a baby grows during pregnancy.
The liver is the largest organ in the body. Its main jobs are to secrete bile (to help digestion), detoxify the blood and change food into energy.
Heart attack
A heart attack happens when there is a blockage in one of the arteries in the heart.
Ovaries are the pair of reproductive organs that produce eggs and sex hormones in women.

Page last reviewed: 13/07/2011

Your GP or nurse can advise you, based on your medical history, whether an IUS is the best method of contraception for you.

Before you have an IUS fitted, you will have an internal examination to determine the size and position of your womb. This is to make sure that the IUS can be positioned in the correct place.

You may also be tested for any existing infections, such as sexually transmitted infections (STIs). It is best to do this before an IUS is fitted so that any infections can be treated. Sometimes, you may be given antibiotics at the same time as an IUS is fitted.

It takes about 15 to 20 minutes to insert an IUS. The vagina is held open, like it is during a cervical smear test, and the IUS is inserted through the cervix and into the womb.

The fitting process can be uncomfortable and painful for some women, especially if you have not had children and your cervix is not stretched. You may also experience cramps afterwards.

You can ask for a local anaesthetic or painkillers, such as ibuprofen, before having the IUS fitted. Discuss this with your GP or nurse beforehand. An anaesthetic injection itself can be painful, so many women have the procedure without.

Irregular bleeding and spotting are common in the first six months after having an IUS fitted.


Once an IUS is fitted, it will need to be checked by a doctor after three to six weeks to make sure everything is fine.

Speak to your GP or nurse if you have any problems after this initial check or if you want the device removed. Also speak to your GP if you or your partner are at risk of contracting an STI as this can lead infection in the pelvis.

Feeling unwell after having an IUS fitted

If you feel unwell, have pain in your lower abdomen, have a high temperature or a smelly discharge after having an IUS fitted, see your GP or go back to the clinic where it was fitted as soon as you can. You may have an infection.

How to tell if an IUS is still in place

Once it is fitted, an IUS has two thin threads that hang down a little way from your womb into the top of your vagina. The GP or nurse that fits your IUS will teach you how to feel for these threads and check that the IUS is still in place.

Check your IUS is in place a few times in the first month and then after each period or at regular intervals.

It is very unlikely that your IUS will come out, but if you cannot feel the threads or if you think the IUS has moved, you may not be fully protected against pregnancy.

See your doctor or nurse straight away and use extra contraception until your IUS has been checked. If you have had sex recently, you may need to use emergency contraception.


Your partner should not be able to feel your IUS during sex. If he can feel the threads, get your GP or nurse to check that your IUS is in place. If you feel any pain during sexual intercourse, go for a check-up with your GP or nurse.


The uterus (or womb) is a hollow, pear-shaped organ in a woman where a baby grows during pregnancy.
Ovaries are the pair of reproductive organs that produce eggs and sex hormones in women.

Page last reviewed: 13/07/2011

Your IUS can be removed at any time by a trained doctor or nurse.

If you are not going to have another IUS put in and you do not want to become pregnant, use another contraceptive method (such as condoms) for seven days before you have the IUS removed. Sperm can live for seven days in the body and could fertilise an egg once the IUS is removed.

As soon as an IUS is taken out, your normal fertility should return.

If you want to start trying for a baby after removing an IUS, ask your doctor or nurse for advice beforehand. You may want to start taking folic acid, stop smoking and cut down on alcohol to improve your chances of conceiving and having a healthy pregnancy.

Page last reviewed: 13/07/2011

Advantages of an IUS

  • It works for five years.
  • It is one of the most effective forms of contraception available.
  • It does not interrupt sex.
  • An IUS may be useful if you have heavy, painful periods because your periods usually become much lighter and shorter, and sometimes less painful. They may stop completely after the first year of use.
  • It can be used safely if you are breastfeeding.
  • It is not affected by other medicines.
  • It may be a good option if you cannot use oestrogens (hormones) such as those used in the combined contraceptive pill.
  • Your fertility will return to normal when the IUS is removed.

There is no evidence that IUDs affect body weight or that having an IUD fitted will increase the risk of cancer of the cervix, endometrium (lining of the womb) or ovaries. Some women experience changes in mood and libido, but these are very small.

Disadvantages of an IUS

Your periods may change. For example, they may become lighter and more irregular or, in some cases, stop completely.

  • Some women experience headaches, acne and breast tenderness after having the IUS fitted.
  • An uncommon side effect of the IUS is the appearance of small fluid-filled cysts on the ovaries. These usually disappear without treatment.
  • An IUS does not protect you against sexually transmitted infections (STIs) so you may have to use condoms as well when having sex. If you get an STI while you have an IUS fitted, it could lead to pelvic infection if it is not treated.

The most common reasons why women stop using IUS are vaginal bleeding and pain. A less common reason is hormonal problems.


Pain is an unpleasant physical or emotional feeling that your body produces as a warning sign that it has been damaged.

Page last reviewed: 13/07/2011

Most complications with IUS happen in the first year after it has been fitted.

Pelvic infections

Pelvic infections may occur in the first 20 days after the IUS is fitted.

The risk of infection from an IUS is extremely small (less than one in 100 in women who are at low risk of STIs). A GP will usually recommend an internal examination before fitting an IUS to be sure that there are no existing infections.

If you have any pain in your lower abdomen or have a high temperature or smelly discharge in the first three weeks after your IUS is fitted, see your GP immediately. It could be pelvic inflammatory disease (PID), which can lead to infertility.


Occasionally the IUS is rejected by the womb (expulsion) or it can move (displacement). This is not common and is more likely to happen soon after it has been fitted. Your doctor or nurse will teach you how to check that your IUS is in place.


In rare cases, an IUS can go through (perforate) the womb or neck of the womb (cervix) when it is put in. This can cause pain in the lower abdomen but does not usually cause any other symptoms. If the doctor or nurse fitting your IUS is experienced, the risk of perforation is extremely low.

If perforation does occur, you may need surgery to remove the device. Contact your GP straight away if you experience a lot of pain after having an IUS fitted. Perforation should be treated immediately.

Ectopic pregnancy

If the contraception fails and you become pregnant, your IUS should be removed as soon as possible if you are continuing with the pregnancy. There is a small increased risk of ectopic pregnancy occurring if a woman becomes pregnant while using an IUS.


Discharge is when a liquid, such as pus, oozes from a part of your body.
Pain is an unpleasant physical or emotional feeling that your body produces as a warning sign that it has been damaged.
The uterus (or womb) is a hollow, pear-shaped organ in a woman where a baby grows during pregnancy.
Ovaries are the pair of reproductive organs that produce eggs and sex hormones in women.

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

Browse Health A-Z