FYI You Can Still Get Pregnant on Birth Control

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We all know someone, don’t we? That friend who was totally on birth control but got pregnant anyway? That person whose story has become something of a cautionary tale?

We’ve always known that hormonal contraceptives are not 100% effective. The warning is drilled into us in sex ed. But why birth control fails when it fails has not been clear. Typically it’s thought to be user error — for example, a woman not using it correctly or reliably, like not taking the pill at the exact same time every day.

But now a new study published in the journal Obstetrics & Gynecology suggests that these oopsie pregnancies may not always be about user error — or serious statistical fluke. Turns out it may be a matter of DNA. That is, when women on hormonal birth control get pregnant, it may be because they carry a gene that renders the hormones in the contraceptives less effective.

A study at the University of Colorado’s School of Medicine looked at 350 women in their early 20s with a contraceptive implant (aka an IUD) that had been in place for one to three years. Five percent of these women were found to have a genetic mutation that lowers the effects of hormonal birth control.

The gene, called CYP3A7*1C, is typically active in fetuses but turns off in infancy. But occasionally it doesn’t turn off, and a woman can end up making this enzyme into adulthood. That enzyme breaks down the hormones in birth control, especially low-dose birth control, and, as a result, puts the woman at higher risk for pregnancy.

This finding is a big deal, not only for the data itself, but for what it confirms about how a person’s genes affects his or her response to drugs. The field of pharmacogenetics (pharmacology = the science of drugs, and genomics = the study of genes) is a new-ish one, but we’re already seeing cancer patients, for example, undergoing more DNA testing in hopes of receiving more targeted treatments. Moving forward, new knowledge in this field will help researchers come up with more tailored approaches to all kinds of health problems, including Alzheimer’s.

And this finding explains something about those those crazy-making conversations we’ve had over the years with our friends, sisters, etc, where one person’s holy grail birth control is another person’s personal hellscape of gnarly side effects. Now we know that due to our unique genetic makeup, we probably really are experiencing hormonal treatments differently. Each of us metabolizes hormones in a unique way.

So for those using hormonal contraception, now you know, and knowing is half the battle. As they say. The other half of the battle? Well, screening will definitely be an important next step. Its accessibility — cost-wise and otherwise– is something we’ll be watching to see how it unfolds.