The contraceptive implant, called Nexplanon (the only one available in the UK) is a small tube that is put under the skin which contains a progestogen hormone.
This hormone is released slowly into the bloodstream and works mainly by stopping ovulation (the release of the egg from the ovary). It also thickens the mucus made by the cervix which forms a 'mucus plug' in the cervix. This stops sperm getting through to the uterus (womb) to fertilise an egg. It also makes the lining of the uterus thinner. This means that if an egg was to fertilise, it is not likely to be able to attach to the uterus.