Your Health Checkup: Why You Can Still Get Pregnant on Birth Control Pills

Five percent of women may carry a genetic variation that could cause their contraception to fail, even if they never miss a dose.

Close-up of a pregnant woman with her hands on her belly

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“Your Health Checkup” is our online column by Dr. Douglas Zipes, an internationally acclaimed cardiologist, professor, author, inventor, and authority on pacing and electrophysiology. Dr. Zipes is also a contributor to The Saturday Evening Post print magazine. Subscribe to receive thoughtful articles, new fiction, health and wellness advice, and gems from our archive. 

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“I can’t be pregnant. I took birth control pills.”

Yes, I’m afraid you can.

As any birth control expert will tell you, no form of contraception — whether it is an IUD, pill, condom, patch, or ring — is 100 percent effective. However, birth control pills that modulate a female’s hormonal status to prevent conception have been singularly effective for many women over many years. When the pills fail, the conclusion reached is usually that the woman skipped a few doses. But is that really true? Can birth control pills fail even when taken consistently?

Apparently, that can happen. Giving the same hormonal pill to every woman and expecting that each will respond in the same way is the mistake in reasoning. Genes make the difference, as shown in a recent study.

In this study, researchers evaluated 350 women who used a contraceptive implant. The researchers tested the women for genetic variations that might impact the efficacy of the contraceptive, while measuring concentrations of hormones in the blood. They found that a gene called CYP3A7*1C, which is usually active in fetuses and then switches off sometime before birth, continued to produce the CYP3A7 enzyme into adulthood.

The enzyme increases the ability of the woman’s body to metabolize the ovulation-suppressing effects of hormonal birth control, thus reducing its effectiveness. About five percent of women have this genetic mutation and end up with a concentration of the hormone in birth control pills below the threshold necessary to consistently suppress ovulation. This would explain how a woman could become pregnant despite taking contraceptive pills. A genetic analysis would reveal whether a woman falls in that five percent group.

Precision medicine is the term for a new approach to medical care designed to optimize efficiency and therapeutic benefits for particular groups of patients. It does so by recognizing and incorporating individual variations in genes, environment, and lifestyle to better focus evaluation and treatment strategies for the individual patient.

The concept of one-size-fits-all has been the approach for many years in which evaluation and treatment are designed for the “average” person, based on the presumption that all will respond in the same way. Precision medicine permits health care workers to personalize treatments based on a more accurate prediction of which approach will work for a specific patient. Precision medicine using genetic analysis is widely used for cancer treatments and to evaluate some cardiac patients, but has only recently been applied to women using birth control pills.

The National Institutes of Health (NIH) has launched an initiative to expand precision medicine in areas such as cancer treatment with the goal to increase knowledge of the genetics and biology of cancer to help find new, more effective treatments for various forms of this disease. The long-term goal is to apply precision medicine to all areas of health and healthcare on a large scale. The NIH plans to launch a study, known as the All of Us Research Program, involving at least one million volunteers from around the United States. Participants will provide genetic data, biological samples, and other information about their health. Researchers will use these data to study a large range of diseases to better predict disease risk, understand how diseases occur, and find improved diagnosis and treatment strategies.

Medicine is on the threshold of a new paradigm of medical care that will provide more precise, individualized approaches to evaluate, diagnose, treat, and prevent diseases. Doctors will gain a better understanding of the underlying mechanisms by which various diseases occur and be able to use patients’ genetic and other information as a part of routine care. It is hoped that precision medicine will improve health care for Americans and people around the world.

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