A Major New Study Reveals Why Hormonal Birth Control May Sometimes Fail

If you’ve ever wondered why hormonal birth control isn’t 100 percent effective, a new study may explain why. Published in the journal Obstetrics and Gynecology, the research found that people who have a certain genetic variant might produce an enzyme that negatively interacts with the hormones in birth control, potentially making it less effective at preventing pregnancy. According to these findings, this is why hormonal birth control methods carry a slight margin of failure, and why some people can still get pregnant while using them. Essentially, hormonal birth control failure may be a result of a genetic mutation — but the good news is, the variant is probably pretty rare.

The study is the first to take a look at how genes interact with birth control, ob-gyn and lead study author Aaron Lazorwitz, MD, assistant professor of Obstetrics and Gynecology at the University of Colorado School of Medicine said in a recent press release on the research. "The findings mark the first time a genetic variant has been associated with birth control," he said. While it’s well known that some people still get pregnant even though they use hormonal contraception, researchers haven’t fully understood why. According to the press release, people are often blamed for using their birth control incorrectly, but the new research sheds more light — and less blame — on the issue.

Lazorwitz told Susan Scuttl writing for CNN that most doctors are familiar with the problem of patients getting pregnant on the pill. He said that patients will often say that “I was taking my pill perfectly. I took it every day and got pregnant, but no one knows why.”

“The kind of historic viewpoint has been that they must have missed the pill; they must have done something wrong,” Lazorwitz said. That viewpoint, however, didn't sit well with him. “I started talking with a pharmacogeneticist at the University of Colorado, and came upon this fascinating area that hadn’t yet been applied really to women’s health care or at all to birth control.”

According to Megan Molteni writing for Wired, people who have this genetic variant may produce an enzyme that “eats away at the ovulation-suppressing effects of hormonal birth control,” leaving them potentially vulnerable to an unintended pregnancy, through no fault of their own. Additionally, two more common genes may have a similar effect on your birth control, though their impact is probably pretty marginal, Molteni wrote.

For this research, the study’s authors enlisted 350 female participants. The median age of the subjects was 22 and a half. Each study subject had a contraceptive implant in place for between 12 and 36 months, according to the press release. (Planned Parenthood says that this kind of implant is more than 99 percent effective at preventing pregnancy, compared to 91 percent effective for typical use of the pill.) Researchers found that about five percent of the people studied had a gene that is active in fetuses, but usually gets switched off before birth. For some people, however, this gene may remain active throughout adulthood, making hormonal birth control potentially less effective for them. Moreover, researchers say that, moving forward, this variant could be found during genetic screenings.

“As more genetic data becomes available, clinicians may need to consider adding genetic predisposition to increased steroid hormone metabolism in their differential diagnosis for unintended pregnancies in women reporting perfect adherence to hormonal contraceptive methods," Lazorwitz said according to the press release. By developing more precise medical interventions that are actually tailored to your unique genetic profile, safer and more effective medications — including birth control — may become possible in the future.

For now, if you’re concerned about the potential of getting pregnant even though you’re taking birth control, a backup method, like condoms, can help prevent pregnancy, in addition to protecting against STDs. Just keep in mind that this genetic variant is most likely pretty rare.