An intrauterine system (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 is similar to the natural hormone progesterone that's produced in a woman's ovaries. This prevents pregnancy by stopping fertilised eggs from becoming embedded in the womb.
It does this by causing the womb lining to become thinner and 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.
The IUS that's available in the UK is called Mirena. It works for up to five years after being fitted. If you're 45 or older when you have the IUS fitted, it can be left until you reach menopause or you no longer need contraception.
How do I get an IUS fitted?
Before fitting the IUS, your GP or clinician will perform an internal examination to check the position of the womb. You may also be tested for infection and offered antibiotics.
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.
Some women find the procedure uncomfortable and painful, so you may be offered painkillers. And you may feel some cramping afterwards, but this will pass.
Read more about having an IUS fitted.
Who can use an IUS?
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. An IUS may not be suitable if you're already pregnant, have an untreated sexually transmitted infection or have problems with your uterus or cervix. Read more about when an IUS can be used.
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.
Removing an IUS
Your IUS can be removed at any time by a trained doctor or nurse.
If you're not going to have another IUS put in and you don't 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.
How effective is an IUS?
The IUS is more than 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 safe and effective as a long-lasting method of contraception. Once it’s in place you won’t have to remember to take or use contraception to prevent pregnancy. It can also make your periods lighter, shorter or even stop them completely after the first year. This may help women with heavy periods or painful periods.
However, there are some disadvantages to an IUS as some women will experience headaches, breast tenderness and acne. You will also need to use other types of contraception to prevent sexually transmitted infections (STIs). Read more about the things you should consider before getting an IUS.
In rare cases there may also be some complications after having an IUS fitted. This can include pelvic infections in the first 20 days after fitting, or the IUD moving out of place and needing to be checked. Read more about the risks of having an IUS fitted.
