HealthDay News — More U.S. women get pregnant using short-acting birth-control methods such as pills, patches and vaginal rings than with long-term contraceptive methods, including intrauterine devices (IUDs) and implants, study results show.

“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,” Brooke Winner, MD, from Washington University in St. Louis, and colleagues reported in New England Journal of Medicine.

They conducted a prospective study involving 7,486 participants who were provided with reversible contraception of their choice at no cost, and compared failure rates in long-acting reversible contraceptives (IUDs, hormone shots and skin implants) and other commonly prescribed contraceptive methods (pills, patches and vaginal rings). A total of 334 unintended pregnancies were identified during the study period.

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Among approximately 1,500 women who chose to use a contraceptive pill, patch or ring, between 4% and 5% became pregnant while using those methods each year, compared with 0.3% among the 5,800 women who opted for an IUD or skin implant.

That’s an unplanned pregnancy rate of about one in 330 for those using long-term contraceptive methods. Although, just 176 participants chose to get the depot medroxyprogesterone acetate injection (Depo-Provera, Pfizer) every three months, rates of unintended pregnancy were similarly low, with just two unintended pregnancies occurring during the study period.

“The effectiveness of long-acting reversible contraception is superior to that of contraceptive pills, patch or ring and is not altered in adolescents and young women,” the researchers wrote.

Unintended pregnancy was almost twice as high among participants aged younger than 21 years of age, who used the pill, patch or ring, they noted.

“These data underscore the potential benefits of offering adolescents long-acting reversible contraception (which does not require daily, weekly, or monthly compliance) to reduce unintended pregnancies in this high-risk age group.”

One researcher received compensation for consultation and expert testimony pertaining to vaginal rings; several  disclosed financial ties to pharmaceutical and health care companies.

Winner B et al. N Engl J Med. 2012; 366:1998-2007.