Comedians joke about jean skirts and certain styles of facial hair unintentionally acting as birth control. But sophisticated hormonal birth control can do the opposite: fail when you want it to work. This has more serious consequences than a missed connection due to an unappealing outfit or Fu Manchu. In 60 years of hormonal birth control being prescribed, most any glitch has been attributed to the woman taking The Pill or relying on an implant for birth control doing something wrong. But as of March 2019, up-to-date obstetricians and gynecologists will have to check that assumption. A study published in the Obstetrics & Gynecology journal this month found that hormonal birth control can be derailed by a rare gene present in around 5 percent of women.

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While the discovery may not immediately reduce the number of unintended pregnancies, it is bound to encourage gynecologists and obstetricians to offer women more individualized treatment in the long-term, the study’s authors hope. Here’s how the study upended some common attitudes towards women and hormonal birth control, along with its key findings:

How The Study Got Started

Many millions of women can attest that one can get pregnant while taking hormonal birth control. There has always been the risk that it will fail due to couples not properly following dosage directions, or antibiotics interfering with the drugs. But other incidents of birth control failure have long been considered a mystery. Society and the medical community also tend to consider the unexplained failure rate as an acceptable degree of risk for couples who want an active sex life without getting pregnant.

This recent study eliminates at least some of the mystery surrounding women who get pregnant while taking birth control. It also refutes the idea that all women who get pregnant while they’re taking hormonal birth control are somehow irresponsible (and should head straight to the next taping of a daytime reality show to get all the relationship issues sorted out). Inspired by pharmacogenomics research in cancer and heart drugs, a team of researchers at the University of Colorado School of Medicine studied a group of 350 healthy women in their reproductive years. Each had used the most reliable form of hormonal birth control, an etonogestrel implant under the skin, for a period between one and three years. None had used hepatic enzyme inducers or inhibitors at the same time. They didn’t study women on the pill, which omitted the need to account for potential user error.

The CYP3A7 gene: Not a Star Wars character

What the research revealed was a genetic mutation that can make hormonal birth control fail. It turns out the five percent of women who have the gene produce extra amounts of an enzyme that eats away at the hormones in birth control pills and implants, making them less effective. (Two other common genes that turned up in the study also affected the efficacy of the hormones in prohibiting a woman’s eggs from getting fertilized on a more minor scale.)

This gene that can trigger unwanted pregnancies has a name: CYP3A7. While it’s active in fetuses, it usually stops working before a baby’s born. Women who still have this gene as adults tended to have lower blood levels of the hormones they were counting on for consistent ovulatory suppression. The researchers also theorized that the contraception-cursing effect of  CYP3A7 is even stronger if women were on low-dose hormonal contraceptives.

The GeneThat Makes Birth Control Fail Might Lead To Precision Medicine

The precision medicine that employs DNA tests to help health care professionals develop individualized treatment regimens has not reached women’s reproductive health yet. At the moment, just one contraceptive is on the FDA’s list of 232 drugs that influence those who take it via a known genetic factor, for example. The oversight hurts, since precision medicine minimizes side effects and maximizes results for individuals. The team that identified the CYP3A7 gene would like to see the finding pave the way for precision medicine to enter the realm of obstetrics and gynecology.

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That’s probably a long, expensive way off, but women and their partners can take heart from the team’s goals. They include expanding the study pool to a much larger cohort and adding whole genome sequencing, along with sleuthing how DNA and birth control side effects are linked. As for determining the impact of clothing and facial hair on preventing pregnancy, safe to say the team has better uses for their time.