“How did I fall pregnant? I’m on the pill!” Women finally have some clarity on a question that has baffled them for years. A new study has found that some women may have genes that break down the hormones in contraceptives more quickly, thus reducing the efficacy of their contraceptive.

Contraceptives are not 100% effective. The contraceptive pill, for example, is generally believed to be 99% effective but reasons why contraceptives fail have remained ambiguous.  

When women fall pregnant while on contraceptives, it is largely assumed that they took their pill incorrectly – either by missing it or taking it too late or at erratic times – or were already in the early stages of pregnancy when they first started taking it.

The study’s lead author, Dr Aaron Lazorwitz from the University of Colorado’s School of Medicine, says: “These findings show that we should listen to our patients and consider if there is something in their genes that caused this.”

The researchers examined 350 women with a median age of 22.5 years who were using the contraceptive implant, one of the most effective contraceptive methods available to women, for between 12 and 36 months.

User error on the implant is next to impossible. Because the contraception method is implanted in the woman’s arm, steadily releasing hormones, she cannot forget to take it or take it incorrectly.

The researchers found that 5% of the women they examined in the study had a gene called CYP3A7*1C. While this gene is active in foetuses, it is usually switched off before birth. In a small number of cases, this gene is not switched off and it may result in the production of an enzyme called CYP3A7.

For the 5% of women who still had this gene, the researchers found the enzyme it produces breaks down the hormones in birth control. This may be putting them at risk of pregnancy while using contraception, especially when using contraceptives with a low dosage of hormones.

Lazorwitz said the findings mark the first time a genetic variant has been associated with birth control. According to him, the study may motivate the development of precise medical tools that can tailor treatment to individual patients.