Reviewed September 2019

Considering the contraceptive implant? Here’s how it works

Considering the contraceptive implant here's how it works

With so many types of contraception available, and so many unfamiliar names of hormones being thrown around, choosing a type which works for you can often be overwhelming. Some women decide to go with the pill, yet remembering to take it can be a struggle. Others may decide to get a coil inserted, a procedure that sounds too scary for some to go through with. However, if you’re looking for a form of long-term contraception which isn’t the coil, there are options out there. Here we take a look at the contraceptive implant and explain how it works to prevent pregnancy, as well as detailing some potential side effects.

The contraceptive implant belongs in a group called long-acting reversible contraceptives (LARCs) which also includes coils and injections; the use of these are often encouraged as they prevent so-called “user failure”, in other words: forgetting to take your pill. It comes in the form of a small piece of plastic which is implanted under the skin in your arm.

Having a contraceptive implant inserted is a quick procedure which can often be done at your GP surgery, or at a local GUM clinic. If you’re thinking of having one, your doctor will be able to explain what will happen in more detail. To summarise: an area of skin on the inside of the upper arm is cleaned and from then on the procedure is sterile; the person doing the procedure will wear sterile gloves and all equipment will be sterile. This reduces the risk of introducing any infection.

Local anaesthetic is injected into the skin in the area where the implant will be inserted; this can sting a bit and is usually the worst part. Soon after, the patch of skin should be numb and there might be a sensation of pushing, but no pain. A small cut is made in the skin as an entry for the 4cm long implant, which is then inserted superficially under the skin of the arm using a device. Usually a couple of steri-strips will be used to close the small cut in the skin.

There is likely to be some bruising for a few days after the procedure. The implant can be felt under the skin, and can stay in for up to 3 years. After this, removal is usually straightforward: local anaesthetic is injected into the skin, a small cut is made, and the implant is pulled out.

How does the implant work?

The above information is relatively easy to find, but a question many women ask is how a piece of plastic under the skin in the arm works to prevent pregnancy. Nexplanon is the brand of contraceptive implant used in the UK (it is the bioequivalent of Implanon, which was previously the only implant available in the UK); it contains etonogestrel, a form of the sex hormone progesterone, dissolved in a membrane inside the plastic rod. The membrane controls the gradual release of the hormone over three years.

Progesterone is a major hormone in the normal female reproductive cycle, and it has a number of roles. It is produced in the second half of the menstrual cycle, after ovulation (the release of an egg from the ovaries) and also has a major role in pregnancy.

The etonogestrel in the implant works to prevent pregnancy in three major ways: it stops the body from ovulating, it causes thickening of the cervical mucus, and it changes the lining of the womb.1

In a normal menstrual cycle, a number of immature eggs develop in the ovaries. This process starts during the period and continues to around the middle of the cycle, where one dominant egg is released following a surge in a hormone called luteinising hormone (LH) from the pituitary gland (a hormone-controlling gland in the brain). The egg is then available to travel down the fallopian tube to be fertilised by a sperm.

After the egg is released, progesterone is released and sends messages to the brain telling it not to produce more LH. This is beneficial because if there was a further LH surge another egg could be released, which could lead to two pregnancies at the same time. Progesterone continues to be produced throughout pregnancy to ensure that further eggs are not released, which allows the single pregnancy to progress safely and the growing embryo to access the nutrients it needs.2

Etonogestrel works just like the body’s natural progesterone to inhibit ovulation; in some ways the steady release from the implant is similar to the state of progesterone released in pregnancy. Without an egg to be fertilised, a pregnancy cannot occur.

Progesterone released during pregnancy also protects the growing embryo by changing the mucus produced by the cervix (the entrance to the womb) to make it thick and sticky. This means that bacteria, which could be harmful to the pregnancy, are less likely to make their way through into the womb. Etonogestrel also thickens cervical mucus by reducing its water content and increasing its protein content. This causes a physical obstruction to the sperm, making it difficult for them to travel through the cervix and into the womb and meaning that even if an egg has been released, the sperm cannot reach it.3

Finally, the changes to the lining of the womb caused by etonogestrel mean that even if an egg were to be released and a sperm was able to get into the womb and travel to the fallopian tube to fertilise it, the embryo would be unable to implant into the lining of the womb and develop.4

Risks and side effects

Of course, with any form of contraception or medicine there are potential problems. There are a few small risks with the insertion of the implant; occasionally a person may have an allergic reaction to local anaesthetic and with any cut to the skin there is a risk of bleeding and a possibility of infection being introduced, although the procedure is carefully done under sterile conditions. There is also a very slight risk of contraception failure, although Nexplanon is considered to be highly effective — less than one in 1,000 women using the implant will get pregnant, over three years.5

The absence of oestrogen in the implant means that there isn’t an increased risk of blood clots as there is with the combined pill, and evidence suggests there also isn’t a negative effect on cardiovascular disease risk.6 There is also evidence that fertility returns quickly after the implant is removed.7

The effects of the implant on periods are variable and range between having no periods (around 21% of women), to having frequent, heavy periods.8 The individual effect it might have is very unpredictable: less than a quarter of women using the implant will experience regular bleeding.9

The effects on bleeding are not only due to the artificial progesterone being produced from the implant, but the combination of this with the natural hormones produced by the body, including oestrogen. So, whether or not you experience increased or decreased bleeding partly depends on how much of the other hormones your body produces, which varies from person to person.

A common concern is the effect your chosen contraception will have on your weight. However, there is little evidence demonstrating a link between Nexplanon and weight change.10 There is also no evidence to suggest that women are more likely to suffer from headaches.11 While some women report new onset of acne while using the implant, others notice their skin improving, so again, you may or may not experience changes to your skin.

Overall, the contraceptive implant is a relatively reliable form of contraceptive and it is easy to have it put in and removed. A doctor or nurse at your GP surgery or local STI clinic will be able to advise you further and answer any questions you may have.

Featured image is of a person holding out their left arm against a white background

Last updated September 2019
Next update due September 2022

Dr Evie Winther MBChB 

Evie is a GP registrar currently based in Yorkshire. She loves running, cycling, and going to the gym, and is passionate about encouraging involvement and equal opportunities for women in sport. She believes it is hugely important for women to have honest information about their health freely available and be able to ask questions without fear of judgement.

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  1. FSRH, Progestogen-only implants: Clinical effectiveness unit, February 2014, [online], (accessed 20 September 2019)  
  2. Graham, J., and Clarke, C., ‘Physiological Action of Progesterone in Target Tissues’, Endocrine Reviews, vol. 18, no. 4, 1997, pp. 502-519.
  3. Grentzer, J., et al., ‘Use of the Etonogestrel-Releasing Implant’, Expert Rev of Obstet Gynecol., vol. 8, no. 4, 2013, pp. 337-344. 
  4. Maddox, D., et al., ‘Etonogestrel (Implanon), Another Treatment Option for Contraception’, P&T, vol. 33, no. 6, 2008, pp. 337-347.
  5. FSRH, Progestogen-only implants: Clinical effectiveness unit, February 2014, [online], (accessed 20 September 2019)  
  6. Ibid.
  7. Ibid.
  8. Ibid.
  9. Ibid.
  10. Ibid.

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