Long-term contraception more effective than pills

A large real-life test of birth control methods found more U.S. women got pregnant while using short-acting methods such as pills, patches and vaginal rings - and the failure rate was highest when they were used by women under 21.

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.

"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.