Long-acting contraceptives are best at preventing pregnancy

Health experts estimate that half of all pregnancies in the U.S. are unintended. That adds up to about 3 million accidental pregnancies every year – and, as a result, about 1.2 million abortions. About half of these unintended pregnancies can be traced to failures of condoms, birth control pills and other forms of contraception. (In the other half of cases, couples fail to use contraception entirely.)

Which types of birth control are to blame, and why? To find out, researchers from Washington University School of Medicine in St. Louis offered free contraception to 7,486 women and followed them for two to three years to see how they fared.

Women in the study (who were between the ages of 14 and 45) were counseled on the risks and benefits of different methods of birth control – including condoms, vaginal rings, patches, birth control pills, hormone injections, long-acting implants and intrauterine devices – and asked to choose the one they wanted. At any point during the study, they could switch to another method or stop using contraception altogether. Researchers conducted follow-up interviews every three to six months. They also tracked birth control use through pharmacy records and logs the women kept on their own.

When all the data were in, the researchers tallied 334 unintended pregnancies among the group. Of these, 156 resulted from birth control failures.

Compared to women who didn’t have unintended pregnancies, those who did were “younger, less educated, more likely to be black, and more likely to rely on public assistance or to report difficulty paying for basic expenses. They were also more likely to have a history of unintended pregnancy, abortion, or sexually transmitted infection,” according to the study. The average age of women who got pregnant by accident was 23.6, versus 25.2 for women who didn’t get pregnant.

So how did the various methods stack up?

The long-term, reversible methods – IUDs, hormone injections and implants – worked much better than pills, rings and patches, according to results published in Thursday’s edition of the New England Journal of Medicine.

After one year, the pregnancy rate among women using pills, rings or patches was 4.8%; after three years, 9.4% of women using those methods had an unintended pregnancy. Meanwhile, only 0.3% of women using IUDs or implants got pregnant after one year in the study, and only 0.9% got pregnant after three years. The numbers were even better for women who got hormone injections – a 0.1% pregnancy rate after one year and 0.7% pregnancy rate over three years, the researchers found.

Put another way, there were 4.55 unintended pregnancies for every 100 participant-years among women using pills, patches or rings, compared with 0.27 for women with IUDs or implants and 0.22 for women who got hormone injections.

After taking the women’s age, educational background and number of prior accidental pregnancies into account, the researchers calculated that the risk of unintended pregnancy was almost 22 times higher with pills, patches and rings than with long-term birth control methods.

What made pills, patches and rings so unreliable? Most likely, it’s that women didn’t use these contraceptives correctly, the researchers reported. For starters, some women on the pill failed to take it every day. Age was a factor here – women who were 20 or younger were almost twice as likely to get pregnant using pills, patches or rings compared to women who were at least 21 years old, the study found. But there were no age-related differences among women using IUDs, implants or hormone injections.

Given all these stats, the researchers seemed genuinely puzzled that more women weren’t opting for the long-acting methods. Not only were they more reliable, they noted, they were more reliable because women didn’t have to think about using them every time they had sex.

A summary of the study is online here.

Planned Parenthood has lots of information about birth control methods on its website, including this chart comparing the effectiveness of different types of contraception.

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