In a new study published on Wednesday in the New England Journal of Medicine, about 7,500 women and teens in the St. Louis area were allowed to pick from a variety of contraception methods at no cost.

Over the course of the study -- more than three years for women who completed all follow-up interviews -- participants had a total of 334 unintended pregnancies. For the full study, see: bit.ly/KdMCQp

"We found that participants using oral contraceptive pills, a transdermal patch or a vaginal ring had a risk of contraceptive failure that was 20 times as high as the risk among those using long-acting reversible contraception," said the research team, led by Dr. Brooke Winner of Washington University School of Medicine in St. Louis.

Long-term contraception methods include intrauterine devices (IUDs), hormone shots and skin implants.

Previous research had suggested that more women have unintended pregnancies when they use contraception that requires daily or weekly use.

But those findings came from national surveys where women tried to remember when they became pregnant and which contraceptive method they were using three or four years earlier.

"If I try to remember what I had for dinner two days ago, I can't," Winner told Reuters Health. "So you can imagine that those retrospective studies are not very reliable."

In the study led by Winner, some of the unplanned pregnancies occurred when women weren't using their chosen contraception and instead relied on a condom or the pull-out method, in which the male withdraws before ejaculation.

Still, among about 1,500 women who chose to use a contraceptive pill, patch or ring, between 4 and 5 percent became pregnant while using those methods each year.

In comparison, 0.3 percent -- about one in 330 -- of the 5,800 women who opted for an IUD or skin implant had an unplanned pregnancy each year because of contraceptive failure.

Just 176 women chose to get Depo-Provera hormone shots every three months, and two of them became pregnant during the study.

Depending on the brand, IUDs can prevent pregnancy for five to 10 years. Skin implants last for three years.

AGE IMPACTS RATE OF UNPLANNED PREGNANCIES

The effectiveness of the methods varied with age: women under the age of 21 who used pills, the patch or a vaginal ring had almost twice as many unintended pregnancies as older women who used those methods, the researchers reported.

Half of all pregnancies in the United States are now unintended -- about 3 million annually, according to statistics from the Centers for Disease Control and Prevention.

About half of those, in turn, are in women who have been using contraception and either do not use it properly, or the method fails. Of those unintended pregnancies, 1.2 million or so lead to abortions.

The rate of unintended pregnancies in the United States tends to be higher than in other developed nations, the researchers said.

They found that women who asked for pills, a patch or a ring were less likely to have given birth before or have had an unintended pregnancy, abortion or sexually transmitted disease compared with participants who selected other methods.

IUDs and implants usually cost more than $500 and typically aren't covered by insurance, she said.

"Nationally, only about 5 percent are using long-lasting methods like IUDs and implants. We know one of the barriers to why they're not using them more frequently is up-front costs," Winner added.

"One of the beauties of this study is that it shows that when you take cost out of the equation and you educate women objectively and effectively, about 75 percent of them chose a long-acting method," she said.

"If that many women were using these products nationally, there would be a very significant drop in unintended pregnancies, which would have far-reaching effects."

(Editing by Michele Gershberg and Cynthia Osterman)

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