Dr. Eileen Costello, a Boston pediatrician, works at a clinic where midwives can readily put in long-acting methods. But she said, “most of our kids coming in don’t want their parents to know.” That can make them less willing to choose long-acting methods, which can cause bleeding for several months, something mothers might notice, she said.

Younger teenagers, “who we know are at highest risk,” often resist longer-acting methods because IUD insertion can be painful, and “they’re freaked out by the idea of an implanted device in their body,” Dr. Costello said.

The new study, conducted at Washington University in St. Louis, involved teenagers at high risk for unintended pregnancy because they were sexually active or were planning to be within six months. About 18 percent had had an abortion, and 60 percent were using no birth control or using withdrawal or condoms — all methods with failure rates of more than 15 percent, said an author of the study, Dr. Jeffrey Peipert, an obstetrics and gynecology professor.

Dr. Peipert said that because long-acting contraceptives have failure rates of less than 1 percent, they were discussed with teenagers before enrolling and as the first options listed. As further encouragement, the clinic could usually put in the implants or IUDs right away.

The yearly pregnancy rate in the study was 34 per 1,000 teenagers, and the abortion rate was 9.7 per 1,000. Pregnancy and abortion rates for sexually experienced teenagers nationally were far higher — 158.5 and 41.5 in 2008. For all teenagers, the pregnancy and abortion rates were 57.4 and 14.7 in 2010.

Teenagers using long-acting methods also stuck with them longer, most of them for at least two years.

Like most contraceptive methods, long-acting ones do not protect against sexually transmitted diseases. In previously published research involving 7,700 females who were 14 to 45, researchers found little evidence that those who used them became more sexually active and found no indication of increased sexually transmitted diseases.