Intrauterine system (IUS)

An intrauterine system (IUS) is a small, T-shaped plastic device that contains progestogen. It’s inserted into your uterus (womb) by a specially trained health professional. Find out how it prevents pregnancy, and how it might affect your periods.

A woman can get pregnant if a man’s sperm reaches one of her eggs (ova). Contraception tries to stop this happening by keeping the egg and sperm apart or by stopping egg production. One method of contraception is the intrauterine system, or IUS.

The IUS is a long-acting reversible contraceptive (LARC) method. Once it's placed in your uterus it works for five years, so you don't have to think about contraception every day or each time you have sex.

The brand name of the IUS used in the UK is Mirena.

You can use an IUS whether or not you've had children.

How does an IUS work?

The IUS releases a progestogen hormone into the womb. This:

  • thickens the mucus from your cervix, so it's difficult for sperm to move through and reach an egg
  • thins the womb lining so that it's less likely to accept a fertilised egg
  • stops ovulation (releasing an egg) in some women 

How effective is an IUS?

It's more than 99% effective. This means that fewer than one in every 100 women who use the IUS will get pregnant in a year.

Added benefits of an IUS?

  • Like the IUD, it can be taken out at any time by a specially trained health professional. Your fertility quickly returns to normal.
  • The IUS can make your periods lighter, shorter or stop altogether, so it may help women who have heavy periods or painful periods.
  • It can be used by women who can't use combined contraception (such as the combined pill), e.g. those who have migraines.
  • Once the IUS is in place, you don't have to think about contraception every day or each time you have sex.

What else should I know?

  • Some women may experience mood swings, skin problems or breast tenderness.
  • There's a small risk of getting an infection after it's inserted.
  • It can be uncomfortable when the IUS is put in: painkillers can help with this.
  • The IUS can be fitted at any time during your menstrual (monthly) cycle as long as you're definitely not pregnant. Ideally, it should be fitted within seven days of the start of your period because this will protect against pregnancy straightaway. You should use condoms for seven days if the IUS is fitted at any other time.
  • The IUS must be removed by a trained health professional.

By using condoms as well as the IUS you'll help to protect yourself against sexually transmitted infections (STIs).

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