Oral contraceptives are among the most popular forms of birth control in the United States. They are also among the least effective.

That’s according to a study published May 24 in the New England Journal of Medicine, which found that adolescent and adult women who use birth-control pills, vaginal rings or transdermal patches as contraceptives are far more likely to unintentionally become pregnant than those using IUDs or subdermal implants.

The study tracked 7,486 females ages 14 to 45 for three years. The women, all enrolled in a program for those at increased risk of unintended pregnancy, were taught about the risks and benefits of various birth-control methods and then allowed to choose one, which they were provided free of charge. They were able to switch methods during the study; those switches were accounted for in the data analysis.Over the three years, 334 unintended pregnancies occurred.

Women using pills, patches or rings were 20 times as likely to unintentionally become pregnant than those using IUDs or implants.

Among the group using pills, patches or rings, those younger than 21 were twice as likely as the older women to have an unintended pregnancy.

The difference is likely due to the fact that IUDS and subdermal implants, once put in place, do their job without requiring any further attention. (IUDs need to be replaced after five to 10 years, depending on what kind they are; implants should be replaced after about three years.) But women have to remember to take a pill every day, replace their vaginal ring every three weeks or put a new patch on every week.

The study’s introduction notes that about half of all pregnancies in the United States (about 3 million each year) are unintended and that about half of those result from birth-control failure. The other half occur when no birth control is used at all.