Implanted and embedded birth-control devices are among the latest contraceptives to hit the market. Depending on a woman's preference, the small, seemingly invisible pieces can be inserted in the uterus, or even lodged under the skin of the forearm.

Get it and forget it, say many companies and doctors who recommend the devices to busy women who might lose track of a daily pill.

But it's possible, although rare, for the hormonal contraceptive device to go missing in a woman's body, experts say.

It's unclear how many times that has happened in the United States, but many women in the U.K. say they fear infertility after their under-the-skin implanted device, Implanon, got lost, according to an article in the U.K. Daily Mail.

Merck, the New Jersey-based manufacturer of Implanon, declined to go into detail about the device's movement in a women's body, instead citing warnings in the patient handout.

Its literature cites "problems of insertion and removal" as a potential risk of the device.

"Removal of the implant may be very difficult or impossible because the implant is not where it should be," according to Merck's patient handout on the device.

Dr. Alan Penzias, director of the fellowship program in reproductive endocrinology and infertility at Boston IVF, said, "The most likely cause in cases where it was inserted correctly is migration within the deep fatty tissue of the upper arm."

Although safe for women even if lost, they might not be able to conceive until the device is either located and removed, or until the hormone runs out, which can be up to a few years.

"There is no reason to think it would cause permanent infertility," Dr. Lauren Streicher, a gynecologist at Northwestern Memorial Hospital in Chicago, said.

"The "worst-case" scenario would require a slightly more involved procedure to remove the device if it embeds itself more deeply than planned," Penzias said.

The same can be true with intrauterine contraceptives, where the device might embed deeper than necessary. "Infection is not a consequence since the device was placed under sterile conditions," he said.

Toni of Boston, Mass., went to her doctor last year for routine removal of her intrauterine device but her doctor couldn't find it.

"I had to go for an ultrasound so they could make sure it was in the right place," said Toni, 29, who asked that her last name not to used. "I was a bit nervous that it had somehow dislodged and was floating around."

Dr. Streicher, who has never encountered such a case, said, "It is rare for any of these devices to migrate from their appropriate location and it won't just show up at another random location."

In rare cases, intrauterine devices can perforate the uterus and end up in the abdominal cavity, she said, but is unlikely to cause any complications if removed.

In Toni's case, the ultrasound found the device in the correct spot, but her doctor was unable to remove it because he could not find the string to pull the device out, she said. Toni had to undergo surgery to have the device removed.

"I will never get another one," she said. "I'm too scared."

But such anecdotes discourage other women from asking about these contraceptive methods, Streicher said.

"Long-acting contraception that is not user dependent has the lowest pregnancy rates of any reversible contraception and are underutilized in this country," she said. "If anything, women should be encouraged to consider them as an option instead of being steered away."

Penzias of Boston IVF said women have many options in contraception and should discuss the risks and benefits with their doctors.

"With good information in hand," he said, "the wide array of options currently available enables women to make good informed decisions about their reproductive lives."