The Mirena coil v. the Pill

How does it work?
The IUS works in several ways. The direct action of levonorgestrel on the endometrium makes it unresponsive to the body's circulating oestrogen, rendering it very thin and inactive. This prevents implantation of a fertilised egg, and also reduces the amount of blood lost with the monthly period. It also works by making the plug of mucus in the cervix thicker and less penetrable for sperm. In the early months of use there is a small chance that the IUS may dislodge and come out. This happens slightly more frequently than with other coils, because the Mirena is larger. Persisting pain and irregular bleeding would be hints that all was not well. Sometimes there are no signs at all that the IUS has come out. Checking the threads regularly will help alert you to this, as would the return of periods after a time of light or absent bleeding.

The freedom of lighter bleeding
Although the IUS was developed as a contraceptive, it was soon discovered that periods became much lighter. After three months use the average blood loss is 85% less and after 12 months 97%. Up to a third of women will have no periods at all. Women find it a bit unusual not having periods but most accept and enjoy the freedom after a while.

The other advantage is that periods are far less painful because levonorgestrel reduces the amounts of the chemical prostaglandin in the endometrium, the commonest cause of period pain.

How effective is it?
The Mirena is much more effective at preventing pregnancy than copper coils and avoids many of the unwanted effects of other coils and hormonal forms of contraception (oral contraceptives, injections). If 1,000 women used the Mirena for a year, only one would become pregnant. This compares with about ten for the copper IUCD, 20 for the Pill and 10-15 for the contraceptive injection. This is comparable with sterilisation. There are other positive factors: