The research, by investigators at Washington University School of Medicine in St. Louis, appears online Oct. 4 in Obstetrics & Gynecology.

Among a range of birth control methods offered in the study, most women chose long-acting methods like intrauterine devices (IUDs) or implants, which have lower failure rates than commonly used birth control pills. In the United States, IUDs and implants have high up-front costs that sometimes aren't covered by health insurance, making these methods unaffordable for many women.

"The impact of providing no-cost birth control was far greater than we expected in terms of unintended pregnancies," says lead author Jeff Peipert, MD, the Robert J. Terry Professor of Obstetrics and Gynecology. "We think improving access to birth control, particularly IUDs and implants, coupled with education on the most effective methods has the potential to significantly decrease the number of unintended pregnancies and abortions in this country."

Unintended pregnancies are a major problem in the United States. Each year, about 50 percent of all U.S. pregnancies are unplanned, far higher than in other developed countries. About half of these pregnancies result from women not using contraception and half from incorrect or irregular use. The Contraceptive Choice Project enrolled 9,256 women and adolescents in the St. Louis area between 2007 and 2011. Participants were 14-45 years of age, at risk for unintended pregnancy, and willing to start a new contraceptive method.

Participants had their choice of birth control methods, ranging from long-acting forms like IUDs and implants to shorter-acting methods such as birth control pills, patches and rings.

The women were counseled about the different methods, including their effectiveness, risks and benefits. The extremely low failure rate (less than 1 percent) of IUDs and implants over that of shorter-acting forms (8 percent to 10 percent) was emphasized. In all, about 75 percent of women in the study chose IUDs or implants.