By Diana Chiu
A Mirena coil is a commonly used long-acting, reversible form of contraception (LARC) inserted into the womb, that can also be used to prevent heavy periods. Otherwise known as an intrauterine system (IUS), the Mirena is small, plastic, and T-shaped, and it works by slowly releasing the hormone levonorgestrel (a natural progesterone) into the womb.
It prevents pregnancy because it increases the thickness of mucus produced by the cervix, therefore making it harder for sperm to enter the womb and fertilise an egg. On top of that, the Mirena coil itself causes a (harmless) local inflammatory reaction within the womb because the body sees it as an object that does not belong there. This reaction makes the womb an unfavourable environment for pregnancy to occur.
In addition, levonorgestrel thins the womb lining and stops the formation of dilated blood vessels needed for implantation of an egg. Therefore, should an egg end up fertilised, it is less likely to be able to implant into the womb. As the womb lining becomes thin with the Mirena, and the formation of dilated blood vessels reduced, it means that when the lining is shed during periods these are lighter, or even absent, hence why the Mirena is used to treat heavy periods too.
When you place your trust in a form of long term contraception, you may still occasionally have little worrying niggles that it might have stopped working. But will you see any signs if your Mirena coil has stopped working? How can you tell?
Could anything cause my Mirena coil to be ineffective?
When inserted correctly, the Mirena coil is 99% effective at preventing pregnancy, and is therefore one of the most effective forms of contraception available in England.1 Once inserted, it lasts for five years, although there are some very rare instances when it may not be effective for that full length of time.
Causes of Mirena coil failure include the coil being misplaced, expulsion (when womb contractions force the device out), and pregnancy that was missed at the time of insertion of the Mirena.2 Even more rarely, the Mirena may move and embed into the womb or pierce the uterus, although the risk is 2.6 in 1000.3
During the first 3-6 months of having the Mirena inserted, irregular periods (frequent, heavy periods alternating with infrequent, light periods) are common, and does not necessarily mean the coil is not working.
The risk of complete expulsion is 1 in 20, and is most likely to occur in the first three months of insertion, as the womb reacts to this ‘foreign’ object by contracting more forcefully and pushing the coil out.4 Women who have heavy or painful periods, or who have never conceived before, are at greater risk of this happening, although the exact reason for this is not clear. It is likely that women who have heavy periods are more likely to dislodge the Mirena due to shedding from the lining and causing the Mirena to move, if it wasn’t lodged well in the first place.
Due to any of the above reasons, the Mirena coil may fail to work effectively and you are at risk of pregnancy. If pregnancy does occur, there is a slightly increased risk that the fertilised egg will implant somewhere other than the womb lining, known as an ectopic pregnancy. This is because the progesterone in the Mirena slows down the passage of the egg through the Fallopian tube, making it more likely to stop and implant there instead of the womb.
Symptoms that may suggest your Mirena is no longer working as it should
Change in character of periods
When the Mirena coil is in place it generally results in lighter periods or no periods at all, as explained above. If, suddenly, ‘proper’ periods occur after a period of experiencing lighter or no periods, this means that the womb lining has been allowed to thicken normally and shed to produce menstrual bleeding. This could indicate Mirena failure, because the levonorgestrel is not having the desired effect.
However, during the first 3-6 months of having the Mirena inserted, irregular periods (frequent, heavy periods alternating with infrequent, light periods) are common, and does not necessarily mean the coil is not working — it is simply because the body is adjusting to the effect of the Mirena.
One suggestion is that the imbalance in the body between progesterone and oestrogen caused when the Mirena is inserted causes the womb lining to be fragile, and therefore more likely to bleed and cause irregular bleeding.
If there is a sudden change in the bleeding pattern, particularly heavy periods, it is worth getting this investigated as it may be due to other causes such as infection or expulsion of the Mirena.
Resurgence of premenstrual syndrome symptoms (PMS)
The exact mechanism of premenstrual syndrome (PMS) is not fully understood. It has a range of symptoms which are different in different people, which can vary from month to month. These symptoms may include mood swings, bloating, changes in appetite, and changes in sex drive.
Some women report feeling their symptoms of PMS are lessened after they have the Mirena inserted, and their mood is more stable, but others may find they have worse mood swings, or feel more anxious.
Everyone is different and it is not certain how the coil will affect each individual’s mood. However, resurgence of PMS in people who found their symptoms improved with the coil in place could be an indication that the Mirena is no longer working effectively, and it is worth seeing your doctor just in case.
One of the most commonly reported symptoms of women who have the Mirena coil is pelvic cramping. This is common for a few weeks after the coil is inserted and is due to the womb being ‘irritated’ and inflammatory cells stimulating the nerves in the womb. In most instances this will get better with time.
However, severe abdominal pain may be an indication that the Mirena is not working, and could be caused by ectopic pregnancy, pelvic infection, or piercing of the uterus. This may be accompanied by other symptoms, such as abnormal menstrual bleeding, or a fever. If you experience sudden severe pain, with or without these symptoms, you should go and seek medical advice. Pain in the first week after insertion may be due to the womb getting used to the mirena, but severe pain is unusual.
No longer being able to feel the strings of your coil
As explained, if the Mirena moves from its correct position in the womb, it can fail to work because the levonorgestrel it is releasing is not being released into the uterus. To ensure that the Mirena is in the proper position, it’s a good habit to check the threads that are attached to the device, which you will be able to feel at the top of the vagina with your fingers, once a month.
If you feel more than just threads, or cannot feel the threads at all, then the Mirena may not be in the right position, and may therefore not prevent pregnancy.
If you feel more than just threads, or cannot feel the threads at all, then the Mirena may not be in the right position, and may therefore not prevent pregnancy. Use another form of contraception such as condoms while seeking help from your GP or practice nurse to check the Mirena is still in the right place.
Although the Mirena releases levonorgestrel directly into the womb, some of the hormone leaks into the bloodstream. As a result, levonorgestrel can cause symptoms such as breast tenderness. This is because it stimulates the growth of mammary cells, stretching the breast tissue. This may or may not be experienced by someone who has the Mirena, because everyone responds differently to hormones.
Breast tenderness can also go down as the body gets used to the change in hormone levels. However, a sudden change in your breasts, such as the breast tenderness suddenly disappearing, may also indicate a drop in progesterone levels and may be a signal the Mirena is no longer working, so it is worth checking the threads to ensure the coil is in position, and visiting your doctor if you are still concerned.
When your symptoms could be a sign of something else
The Mirena coil is an extremely effective form of contraception and failure to work is rare. A sudden change in the character of your periods, stomach pains, change in mood, or change in breast tenderness are not necessarily symptoms of the coil not working, and may not be due to anything at all apart from your body adjusting to the new balance of hormones.
It is important to seek medical help if you do experience a sudden change in the character of your periods however, particularly if they become very heavy, or if you experience severe abdominal pain or fever. This is because these symptoms could be indicative of infection, ectopic pregnancy, or expulsion of the coil.
If you have had your coil for verfour years and you start noticing changes to your menstrual cycle, such as it becoming more like it was before you had the coil inserted, it may be that your coil is becoming less effective, and that you are due a new one. In general, in the UK, GPs may not send you a reminder to get Mirena coil changed. Therefore, go to your GP or health clinic five years after insertion to get a new coil, if you want to continue using contraception.