The long arm of the Mirena® is covered by a fine membrane. This contains progestogen (a hormone similar to the hormone progesterone which is naturally produced by the female body). Progesterone prevents a pregnancy for up to 5 years by steadily releasing small amounts of the progestogen directly into the uterus. The effect of the hormone is to:

  • thicken the mucus in the cervical canal to prevent sperm entering the uterus
  • change the lining of the uterus, making it unsuitable for pregnancy