Both types of contraception are more invasive than the pill, requiring a doctor to put them in place. That and cost are probably why the pill is still the most popular form of contraception in this country.
But forgetting to take even one can lead to pregnancy, which is why the pill is sometimes only 91 percent effective.
An IUD, or intrauterine device, is a small, T-shaped piece of plastic inserted in the uterus that can prevent pregnancy for up to 10 years. An implant is a matchstick-size plastic rod that releases hormones. It is placed under the skin of the upper arm and usually lasts three years.
The new guidelines don't tell teens not to use other methods, but "if your goal is to prevent a pregnancy, then using an implant or an IUD would be the best way to do this," said Tina Raine-Bennett, head of the committee that wrote the recommendations.
The organization's previous guidelines, issued in 2007, also encouraged the use of IUDs and implants among teenagers. The new guidelines go further in saying physicians should discuss the two types of birth control with sexually active teens at every doctor visit.
The gynecologists group said condoms should still be used at all times because no other birth control method protects against AIDS and other sexually transmitted diseases.
While it may sound surprising that such invasive contraceptives are being endorsed for teenagers, 43 percent of girls ages 15 to 19 have had sex, a government survey found. Most are using some kind of effective birth control, but only about 5 percent use the long-lasting devices, the gynecologists group said.
The IUD and implant cost hundreds of dollars. The new health-care law requires health-insurance plans to cover birth control without co-payments. Also, some publicly funded health clinics offer birth control free or at a reduced cost.
The American Academy of Pediatrics has been more cautious and has not endorsed specific methods of birth control, but it is updating its guidance. Some pediatricians have been reluctant to recommend IUDs for teens, partly because of concerns over infection risks; an older model was blamed for infertility.
The contraceptives on the market today "are extremely safe," said Mary Fournier, an adolescent-medicine specialist at Chicago's Ann & Robert H. Lurie Children's Hospital, who praised the new recommendations. "That is what everybody should be telling their patients."