The Mirena coil not only acts as a contraceptive, but reduces heavy periods as well, says Dr Lesley Hickin

The Mirena IUS (intrauterine system) is like many other types of intrauterine contraceptive devices (IUCDs or coils) in that it has to be fitted by a trained doctor and remains in the uterus for a certain amount of time, after which it has to be changed. It consists of a plastic T-shaped frame and the stem of the T has a tiny reservoir containing 52mg of a progesterone hormone called levonorgestrel. Once fitted, about 20 micrograms a day are delivered to the endometrium (lining of the uterus).

Mirena is licensed for five years' use and after this time needs to be removed and replaced. A very small amount of hormone is absorbed into the bloodstream, but much less than the progestogen-only pill (POP), and rarely leads to side effects. If side effects do occur they are transient (four to six weeks) and include headache, breast tenderness and water retention. It contains no oestrogen and therefore has no risks for those women who cannot take the combined Pill.

When is it fitted?
As with all coils the Mirena should be fitted when you are certain you are not pregnant. It is usually done within a week of beginning a period, since this not only reduces the risk of the uterus expelling the coil, but also irregular bleeding, as the endometrium is thin at this time. It can be fitted immediately after termination of pregnancy, but you should wait six weeks after having given birth.

If it is fitted at other times extra contraceptive methods should be used for seven days. Before the Mirena is fitted the doctor will examine you to make sure the pelvic organs are healthy and normal in size. The risk of pelvic infection is slightly raised for three weeks or so after fitting a Mirena, unlike the copper IUCD, where the risk is much higher, mainly in under-25s, who should have a swab taken to exclude chlamydia infection.

When the IUS has been removed, another can be fitted straight away. The lining of the uterus and fertility returns to normal within a month of removal.

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: