A genetic variant present in 5 percent of the population is tied to substantially lower blood levels of the active ingredient in a hormonal contraceptive implant, researchers reported this week (March 11) in Obstetrics & Gynecology. Two other, more common genetic variations also correlated with lower levels of the hormone, although not by as much. The authors say the finding may help explain why women taking some oral hormonal contraceptives sometimes get pregnant anyway.

“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,” coauthor Aaron Lazorwitz, an obstetrician and gynecologist at the University of Colorado, tells Wired. “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...

Lazorwitz and his colleagues recruited 350 women using a form of birth control that is implanted under the skin, where it slowly releases the hormone etonogestrel into the bloodstream to suppress ovulation. They tested the women’s genes and monitored their blood levels of etonogestrel. People with a mutation that keeps the gene CYP3A7, which breaks down hormones, active into adulthood had lower levels of etonogestrel, on average, than those without the variant. Lower levels of etonogestrel were also associated with higher body mass index, a longer length of time women had been using the implant, and to a lesser extent with variants in the genes NR1I2(PXR) and PGR, which code for steroid receptors. 

The risk of unintended pregnancy while on birth control for women who carry the CYP3A7 variant “cannot be quantified. Not at this point. It's too early,” Lazorwitz tells CNN. He says he doesn’t think the variant will affect how well the implant works, but that it could possibly affect the action of oral contraceptives, which put lower levels of hormones into circulation in the body. “There’s so much we don’t know about birth control. There seem to be things outside of a woman’s control like genetics that could impact how well birth control works,” he says. “And we need to start taking those things in consideration.”

The study provides “a glimpse into the next questions we should be asking as to how we can improve care,” Anne Davis, an obstetrician and gynecologist at NewYork-Presbyterian/Columbia University Irving Medical Center who was not involved in the work, tells Reuters. “Knowing that there is a difference in how people metabolize hormones sets the stage for more research that can help us understand the experiences of women better and that can help us give the right medication to the right patient.”

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