Some women release an enzyme that can break down the hormones from birth control, which may make these contraceptive methods less effective in preventing pregnancy. (Credit: Image Point Fr/Shutterstock)

No form of birth control is 100 percent effective. Now, a new study provides an explanation for why a small number of women who use hormonal contraceptive methods still become pregnant, even if they use them correctly.

A new study published in Obstetrics & Gynecology explains that some women have an uncommon genetic difference that makes hormonal contraception less effective for them. In the paper, researchers at the University of Colorado School of Medicine say that around 5 percent of women carry a gene that makes their bodies produce an enzyme that breaks down the hormones in birth control faster than usual.

The researchers think that the enzyme leaves women with hormone levels that may be too low to prevent pregnancy, particularly among users of low-dose contraceptives. Hormonal contraceptive methods like the pill, implant or injection work by releasing synthetic versions of female hormones, usually estrogen and progestin, that overrides a woman's monthly cycle and prevents ovulation.

Receiving these hormones, ironically, tricks a woman's body into thinking it's pregnant, which stops the release of an egg each month. The hormones also work to prevent pregnancy by thickening the mucus near the cervix, which prevents sperm from reaching the egg.

Gene-Drug Interaction

To learn how a woman’s genetic makeup influences birth control hormones, the researchers examined 350 healthy women with a median age of 22.5 years old who had received a contraceptive implant. This long-lasting birth control device sits under the skin and delivers the hormones necessary to prevent ovulation.

The researchers found that around 5 percent of women tested positive for a genetic variant, called CYP3A7*1C. And among these women, the researchers observed lower levels of birth control hormones in their system. It's thought that the enzyme somehow interferes with the ovulation-suppressing effects of hormonal birth control.

Lead study author Aaron Lazorwitz said that the CYP3A7*1C gene normally shuts off during gestation, before a woman is ever born. But in some women that never happens and evidently impacts how they process steroid hormone-based drugs, like birth control. Better understanding genetic differences in medication effectiveness could be a game-changer in women’s healthcare, Lazorwitz said.

“The field of pharmacogenomics, looking at how genetics affects drugs, has been a hot topic in multiple areas of medicine ... [but] women’s health research has unfortunately not focused much on this field to this point,” he said. “As we use the same types of hormonal medications for so many different treatments in women’s health, the impact of genetics on these medications has huge potential to change how we take care of women.”

According to the Centers for Disease Control and Prevention, 24 percent of women use a hormonal form of contraception like the pill or the implant. Lazorwitz said that many cases of birth control failure come down to user error — such as missing a few pills. But, as this study shows, there are factors outside of a woman's control that can impact birth control effectiveness, and there are probably more to find, according to Lazorwitz.

“We think that genetics is part of the equation, but there likely are other things we haven’t even considered yet,” he said. “This is just the first step in our work to try and figure out this complicated issue. Thankfully, we have extremely efficacious birth control methods like intrauterine devices and the [contraceptive] implant that we know work very well for the vast majority of women.”

Failure Risk

Lazorwitz said the findings likely apply to all forms of hormonal birth control — such as the pill, implant or injection — because the hormones used in these methods are similar and are processed similarly in the body. But future studies are needed to prove this.

The unintended pregnancy risk for women carrying this genetic variant cannot be quantified yet because it’s too early. Because the implant releases "more than enough" hormones needed to prevent pregnancy, Lazorwitz said the variant probably does not impact efficacy of the contraceptive implant.

The researchers are more concerned that the genetic variant could affect the effectiveness of lower-dose hormonal methods, like the pill. For now, Lazorwitz said women should continue to work with their doctors in finding the best birth control method for them.

“We want to reassure women taking hormonal birth control that they don’t need to go get genetic screening or anything like that at this time … We hope that this kind of research will one day lead to enough information that we can develop some tools or screenings to help guide women on their individualized decision-making process in choosing a birth control method,” he said.