The intrauterine system This method of contraception is fitted like an IUD. Find out about its pros and cons from this fpa factsheet.

In this article

What is it? How does it work?

How reliable is it? Can anyone use it?

Where can I get an IUS?

The intrauterine system, or IUS, is a small T-shaped plastic device that slowly releases the hormone progestogen. It's effective for five years. The IUS is a long-acting reversible method of contraception. There's only one IUS available in the UK, called Mirena.

• By making the lining of the womb thinner, so it's less likely to accept a fertilised egg
• By thickening the mucus in the cervix, making it more difficult for sperm to reach an egg
• In some women it stops the ovaries releasing an egg (ovulation); most women who use an IUS will ovulate

The IUS is more than 99 per cent effective. This means using this method, on average fewer than one woman in 100 will get pregnant in a year.

All long-acting reversible methods of contraception are very effective because while they're being used you don't have to remember to take or use contraception.

Myth: The IUS is the same as the IUD
Fact: Not true - the IUD is a plastic and copper device, while the IUS releases the hormone progestogen
Myth: It causes infection or infertility
Fact: Not true - the IUS does not cause infection or cause infertility

How's it used?

The IUS can be fitted up to and including the fifth day of your period. It will be effective immediately. If it's fitted after this time you'll need to use additional contraception for the first seven days.

Sometimes your doctor or nurse will check for any possible existing infection. This will be done before the IUS is fitted. In some circumstances, antibiotics may be given at the same time as fitting the IUS.

The IUS has two soft threads at one end, which hang through the cervix into the top of the vagina. These are so you can check the IUS is in position. You should check these after fitting and then about once a month.

Advantages:

• It works for five years
• It doesn’t interrupt sex
• Your periods usually become much lighter, shorter and less painful, and they may stop completely after the first year of use, so the IUS is helpful if you have heavy, painful periods
• It can be used if you're breastfeeding
• Your normal fertility returns as soon as the IUS is removed
• It can be used by women who cannot use estrogens
• It's not affected by other medicines

Disadvantages:

• Your periods may change in a way that's not acceptable to you
• Some women get acne
• Some women report mood changes, headaches or breast tenderness
• Some women develop small fluid-filled cysts on their ovaries - these aren't dangerous and don't usually need treatment
• There's a very small chance of infection in the first 20 days after the IUS has been put in
• The IUS may come out (explusion) or it may move (displacement), but this is most likely to happen shortly after it has been put in
• The IUS may go through the cervix or womb (perforate) when it is put in, but the risk of this is low when it's put in by an experienced doctor or nurse
• If you do become pregnant, there's a small increased risk of you having an ectopic pregnancy (a pregnancy occurring outside the womb, normally in the fallopian tube), but this risk is less than in women using no contraception

Most women can, but it may be unsuitable if you:

• Think you might already be pregnant
• Have cancer of the womb or ovary
• Have breast cancer now or within the past five years
• Have any other problem with your womb or cervix
• Have an untreated sexually transmitted infection or pelvic infection
• Have migraines with aura
• Have active liver disease
• Currently have thrombosis, heart or circulatory disease
• Have unexplained bleeding from your vagina (for example, between periods or after sex)

Other things to consider

Young women or women who've not been pregnant can use the IUS. Once it has been fitted, you can forget about it until it needs replacing.

The IUS is very good for women with heavy, painful periods and can also be used as the progestogen part of HRT for women going through the menopause. The IUS does not protect you against sexually transmitted infections.

They're free on the NHS from contraception clinics, sexual health clinics or your GP.

Advice and support

Call fpa's helplineon 0845 122 8690.

This article was last reviewed in July 2007.