Intrauterine devices and birth-control implants are somewhat miraculous: Once inserted, they protect against pregnancy (but not STDs) for years. No pills, no shots, no problem.

After years of slow uptake, IUDs and implants are finally catching on among American women. In 2013, about 12 percent of women were using these long-acting, reversible contraceptives, or LARCs, compared with just 2.4 percent in 2002. Some studies credit these contraceptives with playing a major role in the reduction of the teen-pregnancy rate.

But a new report from the Urban Institute think-tank suggests that nearly a third of women are significantly misinformed about LARCs. Using a 2016 survey of 798 women of reproductive age, the authors found that fewer women thought that implants, which have a failure rate of about one in 2,000 women, are “very effective” than think that about the pill, which has a failure rate of about nine in 100 women.

Perceptions of Effectiveness of Birth Control Methods Among Women Ages 18 to 44

Urban Institute

What’s more, women were less likely to think that implants or IUDs were “very safe” than they were to trust in the safety of withdrawal (or “pulling out”) or even the rhythm method. The report found that the women lacking knowledge of LARCs were more likely to be single, non-white, low-income, and to have never been pregnant.