The pill is the reversible option most commonly used by women who practice contraception, but it is not the most effective.
In a sweeping new study, women who used birth control pills, the patch or a vaginal ring were 20 times more likely to get pregnant than those using an intrauterine device or implant.
"The major finding is the magnitude of the difference between the most commonly used methods and these long-acting methods," said study author Dr. Jeffrey Peipert, vice chair of clinical research in the Washington University in St. Louis' Department of Obstetrics and Gynecology.
"We knew that pills have a failure rate, and we knew that the long-acting reversible contraception options were excellent," he continued. "We just didn't know the magnitude of the difference."
In the new study, published Wednesday in the New England Journal of Medicine, researchers provided nearly 7,500 women ages 14 to 45 with whatever contraception method they wanted at no cost. They compared rates of failure of long-acting reversible contraception, which includes IUDs (T-shaped devices inserted into a woman's uterus that prevent sperm from joining an egg) and implants (a small rod inserted into a woman's arm) with those of the birth control pill, patch and vaginal ring.
Overall, the long-acting options were superior to more short-term options, with failure rates of 4.55 per 100 participant-years versus 0.27 in women using IUD or implant.
And the research also revealed variations with age: Women under 21 who used the pill, patch or ring had almost twice the risk of unintended pregnancy as older women using the same methods.
Researchers say the "why" behind such failure rates is simple: human error.
According to Planned Parenthood, fewer than 1 out of 100 women will get pregnant annually if they always take the pill as directed, but nine of 100 will if they don't always take the pill every day, per instructions.
"It's the human factor. We just make mistakes sometimes," Peipert explained. "The long-acting reversible contraception methods are forgettable. You put them in place and they work very well."
Despite this, relatively few women in the United States use IUD or implants. The Guttmacher Institute, which studies sexual and reproductive health, estimates that from 2006-2008, 5.5 percent of all women who practiced contraception in the U.S. used IUD, and fewer still used the implant. Use is much higher in Europe, accounting for more than a quarter of contraceptive users in places like Norway.
"Part of the low use of IUDs in the U.S. is that providers aren't aware of some of the modern, evidence-based indicators for these methods," said Dr. Sarah Betstadt, an assistant professor in the department of obstetrics and gynecology at the University of Rochester, who was not involved with the new study. "They may not be offering women all the options, or their best option."
Dr. Melissa Gilliam, a professor of obstetrics and gynecology at the University of Chicago, said that there is some confusion about the reversibility of such methods, despite the fact that groups like the American College of Obstetricians and Gynecologists call IUDs and implants the most effective forms of reversible contraception available -- safe for use by nearly all reproductive-age women. "There's a lot of misunderstanding about the IUD," she said.
Another factor, Gilliam said, is cost.
"It can cost someone $700 to $800 depending on how it's being paid for, and then there's also the cost of insertion, because it's an actual procedure," she said, explaining that short-term options eventually add up. "Some have full insurance coverage, and some have no insurance coverage."
But the authors of the new study argue that unintended pregnancy is an issue that places a substantial financial burden on families and society. About 3 million pregnancies in the U.S. per year are unintended and half stem from contraceptive failure, according to research cited in the new study.
"In this country, the default birth control has always been birth control pills," said Betstadt. "This study really shows that there are much more effective methods. That's not to say that all women should be using these methods. But women should have the information they need chose what's right for them."
HuffPost Lifestyle is a daily newsletter that will make you happier and healthier — one email at a time. Learn more