The Medicines and Healthcare Regulatory Agency (MHRA) has said that hundreds of women have become pregnant despite having a contraceptive implant fitted.

A total of 584 women have become pregnant whilst using the implant, Implanon, since its launch 11 years ago and there have also been over 1,600 complaints of adverse reactions to the device.

Implanon is a small plastic rod which is inserted into a woman's arm and releases the hormone progesterone, which is supposed to deter a women becoming pregnant for up to three years. The Department of Health estimates that about 1.4 million women have had the implant fitted since it was first licensed in 1999.

The MHRA has said that it has had complaints from doctors and nurses about problems that have been encountered inserting the device and that MSD, which manufactures Implanon, has now replaced it with another, called Nexplanon, which should be more easily inserted by the health professional involved.

Ann Furedi, chief executive of the British Pregnancy Advisory Service, said that all contraceptives have a failure rate, however small and that women need to be aware of this. However, she added that generally implants are "an excellent and usually reliable method of birth control".