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 the UK 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.