How Does the Contraceptive Coil Work?

IUD stands for intrauterine device. An IUD is a small coil which is inserted into the uterus to prevent pregnancy. There are two different types of coil, the copper coil and the hormone coil. Both provide effective contraceptive cover and are very convenient. Find out how they work and whether a coil is the right contraceptive for you.

Dr Simran Deo Medical Editor

Medically reviewed by

Dr Simran Deo

Last reviewed: 22 Nov 2019

What is the Copper Coil?

The copper coil is a small T-shaped plastic coil coated with copper. Like the hormonal coil, it is inserted into the womb, where it releases small copper ions. Copper ions have a similar effect as a spermicide, as they kill sperm. In addition, it is believed that the coil also prevents fertilised eggs from implanting in the womb.The coil has threads attached to it, which allow you to check that it is still in the correct place. The coil needs to be inserted by a trained nurse or a doctor. Your GP will check whether you can use a coil and show you how to check whether the IUD is still in place. A few weeks after you have had the coil fitted, you will need to go for a check-up to make sure you are still happy with the coil.If you want to stop using the coil, you need to see your doctor again to have it removed. Once you have had it removed, you can become pregnant again.

What is the Hormonal Coil?

In addition to coils with a copper coating, there are also coils which work by releasing hormones. They are also known as an IUS, which stands for intrauterine system. The hormonal coil releases progesterone and works in a similar way to the minipill. It does this by thickening the mucus at the entrance of the womb and making the lining of your womb thinner so a fertilised egg won’t be able to implant. It may also prevent your ovaries from releasing mature eggs. Unlike the minipill, you don’t have to remember to take tablets to stay protected.The hormonal coil may also cause similar side effects as the minipill and it is not suitable for everyone.

The Coil as Emergency Contraception

The coil may also be used as an emergency contraceptive. In order to prevent pregnancy from occurring after unprotected sex, it needs to be inserted within five days of the episode of unprotected sex. Not all types of coil however are suitable to be used as emergency contraception. 

How Effective is the Coil?

The most effective copper coil is over 99% effective. It is therefore as effective as the contraceptive pill. However, in actual use it may be more effective, as it is always in place and you don’t need to remember to take any tablets. The copper coil is also not affected by medications or infections. The hormonal coil is also over 99% effective in preventing pregnancy.

Which Side Effects Can a Coil Cause?

The side effects you might experience on the coil depend on whether you are using the copper coil (IUD) or the hormonal coil (IUS). Side effects of the IUD (copper coil)The IUD can cause changes in your periods, with some women experiencing longer or heavier periods. In some cases, women also find that their periods become more painful. Often, these side effects improve after the first few months as your body gets used to the coil.Side effects of the IUS (hormone coil)As with the IUD, the IUS can have an impact on your periods. Some women complain about heavier periods while using a hormonal coil. However, it is also possible that your periods stop entirely or become irregular.The Mirena (hormonal) coil often causes irregular periods during the first 2–3 months. After that, most women find that their periods become lighter or stop completely. If you are worried about having heavy periods, the Mirena coil may be a suitable option for you.As the IUS works by releasing progesterone, some women also experience side effects such as headaches, acne or breast tenderness.

Are There Any Risks to Using an Intrauterine Device?

Intrauterine devices are a safe and very reliable contraceptive but as with most procedures, there is a small risk of complications.InjuryIn rare cases, the coil has been found to perforate the womb during insertion. If you feel a lot of pain after you have had your coil inserted, you need to tell your doctor straight away. If this happens, your coil needs to be removed again immediately.InfectionsIt is also possible that the coil causes an infection. This usually happens within the first few weeks after you have had the coil fitted, and it can cause symptoms such as abdominal pain, a fever and chills or pain on passing urine.Ectopic pregnancyEctopic pregnancy is a rare condition, which occurs when a fertilised egg begins to develop outside the womb. Ectopic pregnancies are rare but very dangerous and need to be treated without delay. Using a coil is associated with a small increase in your risk of developing an ectopic pregnancy if you get pregnant while using it. 

What Happens if the Coil Moves or Falls Out?

Most women who use the coil do not encounter any serious problems but in some cases, the coil can be displaced. This reduces your protection and increases your risk of an ectopic pregnancy. When you have the coil inserted, your doctor will show you how to check whether it is still in place. 

Will I Gain Weight if I Use a Coil?

There is currently no evidence that suggests that using a contraceptive coil, either copper or hormonal, will affect your weight.

Can I Get a Coil on the NHS?

Both types of contraceptive coil are available for free on the NHS. Whether you can get one fitted depends on whether the coil is suitable for you. Your doctor will assess your suitability during your initial consultation and give you detailed information about your chosen contraceptive.

IUD vs. Contraceptive Pill - Which is Better?

Whether the pill works better for you or the copper coil as your chosen contraceptive depends on your health and your lifestyle. If you find it difficult to take the pill at the same time every day, for example because you have an irregular work pattern, a coil may suit you better.

The same applies if you experience side effects on the pill and would like to switch to a hormone-free alternative.

Likewise, if the coil is causing you to experience period pain or irregular periods, you may wish to try a contraceptive pill or the Mirena coil instead.

It is important to note, that neither the coil nor the pill protect you from sexually transmitted diseases. Regardless of whether you are on the pill or have a coil you need to use a condom to prevent you catching an STI.

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