Women who use short-term, reversible birth control methods such as pills, the patch, or ring, are 20 times more likely to have an unintended pregnancy compared to those who used long-term, reversible methods such as intrauterine devices and implants.

Investigators at Washington University School of Medicine also found that short-term, reversible contraceptive methods were especially unreliable among younger women. The risk of unplanned pregnancy was almost twice as high among study participants under 21 who used methods such as birth control pills, the patch or ring as among older women. This finding suggests that use of longer acting contraceptive methods could substantially prevent unplanned pregnancies among adolescents.

Overall, 133 of the women taking pills, or using the patch or ring had contraceptive failure, compared with only 21 of those using IUDs and implants.

The investigators wanted to see if educating women about the effectiveness of various contraceptive options and letting them choose a method no matter what it cost would reduce the rate of unintended pregnancy. Contraception was provided to women free of charge.

More than 7,500 women between the ages of 14 and 45 participated in the study over three years. During the study, 334 women became pregnant, and 156 of these pregnancies were the result of contraceptive failure. Overall, 133 of the women taking pills, or using the patch or ring had contraceptive failure, compared with 21 of those using IUDs and implants.

The women were considered to be at high risk of unintended pregnancy because they were sexually active or planned to become sexually active in the next six months. They told researchers that they were not currently using birth control or wanted to switch methods. The women also said they did not want to become pregnant for one year. Study participants could choose one of the following birth-control methods: IUD, implant, birth control pills, patch, ring, and contraceptive injection. The women were counseled about the different methods and could discontinue or switch methods as often as they wanted during the study.

Women who chose an IUD or implant were more likely to be older, to have public health insurance, and to have more children than women who chose the pill, patch, or ring. Women who chose pills, the patch, or ring were more likely to have private health insurance and be childless. Many women cannot afford the out-of-pocket costs of these methods, which can be more than $500.

"This study is also important because it showed that when IUDs and implants are provided at no cost, about 75 percent of women chose [long-term, reversible] methods for birth control," Brooke Winner, lead author of the study and a fourth-year resident at Barnes-Jewish Hospital in St. Louis, said in a press release.

Investigators interviewed participants about missed periods and possible pregnancy at three and six months and every six months thereafter for the duration of the study. Women who suspected that they might be pregnant were asked to take a urine pregnancy test. Those whose test was positive were asked if the pregnancy was intended and what contraceptive method they were using.

Jeffrey Peipert, senior author of the study and the Robert J. Terry Professor of Obstetrics and Gynecology said in a press release that, based on the study findings, patients should be counseled differently to reduce unintended pregnancy rates. "If there were a drug for cancer, heart disease or diabetes that was 20 times more effective, we would recommend it first," he said. "Unintended pregnancies can have negative effects on women's health and education, and the health of newborns."