By Dr. Jennifer Kelly
If you are reading this article, the chances are that you already have the contraceptive implant, or are thinking of getting one. Contraceptive implants, often known by their brand name Nexplanon, are small, flexible plastic rods that are inserted under the skin in your upper arm. Once inserted, they can remain in place for up to three years.
The contraceptive implant works by steadily releasing the hormone progesterone into your bloodstream. The raised levels of progesterone prevent ovulation (release of the egg) from occurring by inhibiting surges of follicle-stimulating hormone (FSH) and then luteinising hormone (LH) needed to mature and release an egg. Of course, if an egg is not released, pregnancy cannot occur.
The raised progesterone levels can also result in the thickening of the cervical mucus, and thinning of the lining of the womb, both of which would make it less likely for an egg to get fertilised, or implant successfully should ovulation actually take place while the implant is present. This is because progesterone prevents the proliferation of the endometrial lining cells. With continued elevated progesterone levels over time, the lining of the womb slowly atrophies or thins, making it a more hostile environment for a fertilised egg in the unlikely event of ovulation occurring.
How effective is the implant at preventing pregnancy?
The implant is a very effective method of long acting contraception, and while no form of contraception is completely failsafe, using it makes the risk of a woman getting pregnant far less likely.
If a woman has regular intercourse for a year while using no form of contraception, the chance of her becoming pregnant in that year is 15%. So, for every 100 women having intercourse, 15 will become pregnant over the course of one year.1
For women using the contraceptive implant, the chances of getting pregnant are less than 1%, providing the implant has been in place for less than three years. So, for every 100 women with an implant having intercourse, less than one will become pregnant over the course of one year.2
This number is similar to the efficacy of the combined oral contraceptive pill (provided it is taken perfectly, although many women don’t take it as reliably as this).
While your implant is in place, it releases a steady amount of the hormone progesterone. However, the amount it releases decreases slightly year on year. This is why it needs replacing after three years. Research has shown that during this three-year period, for women of a healthy body weight, the implant becomes no less effective at preventing pregnancy however.
In women of a slightly heavier body weight (over 70kg or with a BMI over 30), there remains a slight question of how effective the implant is in the third year of use. Studies have only looked at small numbers. Currently, the manufacturer of Nexplanon states, “It cannot be excluded that the contraceptive effect may be lower than for women of a normal weight.” As such, guidance states in the third year of use of an implant, health care practitioners can consider replacing them if it is thought to be needed, however there is no direct evidence to support this.3
Other methods of contraception including the copper coil, the Mirena coil, and the contraceptive injection, all have similar failure rates to the implant. If you compare this to the withdrawal method, which has rates of pregnancy almost as high as couples using no contraception at all when used typically, you will see why this is not a recommended method of contraception.4
Once the implant has been inserted, perfect use is pretty simple, making it an excellent and almost failsafe form of contraception for many women. Once it is in place there is very little you can do to alter how effective it is, unless you take certain medications while using it (detailed below). In the unlikely event that you stop being able to feel your implant in your arm or it works its way out (which is rare), it is important to use a different form of contraception and see your doctor as soon as you can.
How would I know I was pregnant if I use the contraceptive implant
A missed period isn’t a good indicator of pregnancy when you are using the contraceptive implant. This is because the implant often makes periods lighter or more irregular, and women often find that their periods stop completely (amenorrhoea) while they have the implant. However, some women find the opposite can occur, and they may have heavier, longer periods.
However, on the very rare chance that you become pregnant while using the contraceptive implant, there are a number of signs you could look for. These may include a change in the pattern of your periods (if you still have them), tender breasts, tiredness, or nausea. The easiest and safest way to check if you are concerned you are pregnant is to take a pregnancy test.
If you do become pregnant while using the implant, it is important to see your doctor as soon as you can. Equally, if you develop stomach pain, you must see your doctor urgently to rule out any chance of an ectopic pregnancy. There is good evidence to show that implants do not cause birth defects and will not cause harm to a developing pregnancy. However, it is suggested that in this situation the implant is removed as soon as possible.5
Is there anything I can do to further reduce my chances of getting pregnant on the implant?
The most important thing is to remember to have your implant replaced after three years and to be aware that your risk of pregnancy can be increased if you are taking medications called enzyme inducing drugs. These include certain epilepsy and HIV treatments, and the over the counter remedy St John’s Wort, which people sometimes take to help with symptoms of low mood, depression and anxiety.
These drugs can cause decreased blood levels of progesterone, reducing the implant’s effectiveness. This is because they cause progesterone in the blood to be eliminated more quickly than would normally occur, making progesterone only contraception less effective. While using these medications, it is important to use other contraception and to continue using other contraception for 28 days after you finish using them.6