New research reveals that contraceptive devices are extremely effective for those who do not want a baby.
However, the devices are barely used in developed countries and only a bit more popular elsewhere.
The review analysed studies of three kinds of implants - Implanon, Jadelle and Norplant. The last is currently unavailable.
Researchers tracked women in nine studies for two to five years, and found a total of five pregnancies among the 2,776 participants.
"No one implant was found to be any more or less effective," said study co-author Rebecca French, a senior research fellow with the Margaret Pyke Centre at University College London.
These numbers are not surprising, according to French. Surveys conducted in the United States, which asked women about pregnancies that occurred while they were using birth control, suggest that five in 10,000 women who use implants will become pregnant during a yearlong period, she said. By contrast, U.S. studies suggest that about three in 1,000 women who correctly use the birth control pill for a year will get pregnant, French said; the number is estimated to be eight in 1,000 among women who miss some pills.
"Implantable contraceptives are very effective methods for preventing unplanned pregnancy," French said.
Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.
All of the studied implants prevent pregnancy in the same way. Once inserted under the skin of the upper arm, the implants release the hormone progesterone into the body.
"This thickens the mucous in the cervix, which makes it difficult for sperm to travel through the cervix to reach an egg,. Implants also work by making the lining of the womb thinner, making it less likely to accept a fertilized egg," French said.
Contraceptive implants have been readily available since the 1980s. In 2003, the World Health Organization estimated that some 11 million women worldwide were using the devices.
The review compiles data on nine studies comparing one type of implantable contraceptives to another; none compared implants to other forms of birth control.
Eight of the studies - with 1,578 women total - compared Implanon to Norplant; the makers of Implanon sponsored these studies. Another 1,198 women took part in a single study comparing Jadelle to Norplant.
According to French, there were no pregnancies in the studies that compared Norplant to Implanon. In the one other study comparing Norplant and Jadelle, there were three pregnancies among Jadelle users and two among Norplant users.
Of the three implants, only Implanon is currently available in the United States.
Irregular vaginal bleeding was a common side effect of the implants, but more than 80 percent of women were still using their implant at two years.
"Women in developed countries were much more likely to say that they discontinued using implants because of changes to their menstrual cycle. Few women in developing countries gave this as a reason for discontinuation," French said.
It took less time to remove Implanon and Jadelle when compared to Norplant, and Norplant caused more problems during removal, the review found.
Andrew Kaunitz, professor and assistant chairman at the University of Florida's Department of Obstetrics and Gynecology, said the review findings reflect existing knowledge about the implants.
"They're convenient, they're safe and they're associated with bleeding problems," he said.
Women do not use contraceptive implants commonly in the United States, and Kaunitz said he has only prescribed the devices for five women.
The makers of Norplant decided to stop marketing it in the U.S. in 2002 after multiple complaints about side effects. That negative publicity might explain why American women have not embraced contraceptive implants, Kaunitz said.
Women now have a variety of options for long-acting birth control, including Depo-Provera (given via injection) and contraceptive patches, he added.
The implants are also expensive, costing several hundred dollars each plus the cost of implantation, Kaunitz said. Other female contraceptive methods, like the birth control pill, do not require such a large upfront cost. Though initial costs are high for implants, French said in the long term the devices are cost-effective because they reduce costs associated with unplanned pregnancy.
Kaunitz said that he suspects that implants might become more popular if their manufacturers advertise them. After all, he said, they are actually more effective at preventing pregnancy than surgical sterilization, and the implants are reversible - women can become pregnant again once a clinician removes the implant.
The study appears in the latest issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research.