For nearly 60 years, hormonal contraceptives have freed women from their own biology, giving them autonomy over decisions about if, when, and how to start a family. But no form of hormonal birth control—pill, patch, ring, IUD—is 100 percent effective. Why that is, no one knows exactly. Now new research suggests that some of these mysterious failures might actually be due to differences in DNA.

In a study published today in the journal Obstetrics & Gynecology, researchers at the University of Colorado School of Medicine discovered that about 5 percent of women possess a genetic mutation that makes them produce an extra hormone-dismantling enzyme. This enzyme eats away at the ovulation-suppressing effects of hormonal birth control, lowering its effectiveness. They also found two much more common genes that had smaller but still noticeable effects.

“The biggest takeaway is that we’ve assumed for so long that if a woman taking birth control gets pregnant, then she must have done something wrong,” says Aaron Lazorwitz, an ob-gyn and lead author on the study. “Instead, maybe we need to pay more attention as physicians to other things that might be going on, like genetics, so we can give better, more individualized treatment to women instead of just blindly adhering to the motto that if you just throw some hormones at it, that usually fixes the problem.”

It’s the first time anyone has ever identified unique snippets of DNA associated with birth control performance. Indeed, when Lazorwitz presented his preliminary findings at the North American Forum on Family Planning last October, despite being a last-minute addition with a 7:30 am time slot, he was met with a packed room of doctors and researchers eager to see his results. “It was the only thing like it I’d ever seen,” says Megan Christofield, a family planning adviser at Jhpiego, a Johns Hopkins–affiliated global health NGO, who was in the audience that day. “There was so much energy in the room, everyone was like, ‘Yes! Bring us to this place in science!’”

That place is precision medicine—using a person’s unique genetics to understand which drugs will work best for them. Cancer and heart patients, for example, are increasingly taking DNA tests to determine which treatment will have maximum effectiveness and minimal side effects. But the promise of precision medicine has yet to extend to women’s reproductive health. On a US Food and Drug Administration list of 232 drugs with a known genetic factor influencing how they work, there’s just one contraceptive. And the genetic marker identified doesn’t have anything to do with the drug’s active hormone ingredients. In effect, that means that scientists and doctors have no idea how different women’s bodies respond to the same one or two hormones that make up all birth control drugs.

Instead of genetic tests to figure out which contraceptives would work best for them, women have whisper networks and trial-and-error. Sisters, friends, roommates, and coworkers often make recommendations, but their experiences are not universal. So it’s unsurprising that most American women will have bounced around between three different birth control methods by age 40.

In the US, side effects such as headaches, depression, and unpredictable bleeding might be an inconvenience women are expected to tolerate. But in developing parts of the world, where Christofield works, those side effects can have catastrophic effects on women’s ability to go to school, work, or even leave the house. And they’re often the reason women there stop using contraceptives altogether. Even rumors about side effects sometimes prevent women from accessing hormonal birth control, even if it’s not certain they’ll have the same negative reaction.