The study is published in the New England Journal of Medicine.
Researchers found that long-acting contraception is about 20 times more effective at preventing pregnancy than pills, patches or vaginal rings.
The study involved about 7,500 women in a project promoting long-acting birth control to reduce unintended pregnancies.
"IUDs and implants are more effective because women can forget about them after clinicians put the devices in place," said Dr. Jeffrey Peipert, a professor of obstetrics and gynecology at Washington University.
Failure rates for such methods are less than 1 percent, but require an office procedure and cost more.
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