Has your birth control failed? Blame your genes

Photo: Shutterstock
Photo: Shutterstock 

Have you found yourself expecting a baby, despite using hormonal contraceptives to prevent that happening. Well, genetics might be to blame for your situation.

Women who fall pregnant while on contraceptives may carry a gene that breaks down the hormones common in birth control, according to a new study.

"The findings mark the first time a genetic variant has been associated with birth control," said lead author Dr Aaron Lazorwitz, Assistant Professor of Obstetrics and Gynecology at the University of Colorado School of Medicine.

As part of the study, published in the journal Obstetrics & Gynecology, Dr Lazorwitz and his team examined 350 healthy women who'd had a contraceptive implant for between 12 and 36 months. Researchers chose the implant for ease of study as it meant there were no concerns about women potentially missing a dose. 

The team measured participants' blood concentration of etonogestrel and tested their DNA for specific genetic variants that could affect hormone metabolism. When they analysed the results, they found that five per cent of the women had a gene called CYP3A7*1C. The gene is typically active in foetuses but is switched off before birth. Some women with this gene, however, continue to make the CYP3A7 enzyme into adulthood. In addition, more than one in four women with the CYP3A7*1C genetic variant did not have high enough levels of etonogestrel to prevent ovulation.

"That enzyme breaks down the hormones in birth control and may put women at a higher risk of pregnancy while using contraceptives, especially lower dose methods," Dr Lazorwitz said. "When a woman says she got pregnant while on birth control the assumption was always that it was somehow her fault," he continues. "But these findings show that we should listen to our patients and consider if there is something in their genes that caused this."

According to Dr Lazorwitz, the study highlights the importance of a new field of study -pharmacogenomics, which analyses how genes affect a person's response to drugs. It's an area he says has the potential to dramatically alter the field of women's health.

 "As more genetic data becomes available, clinicians may need to consider adding genetic predisposition to increased steroid hormone metabolism in their differential diagnosis for unintended pregnancies in women reporting perfect adherence to hormonal contraceptive methods," Dr Lazorwitz notes.

Hilary Rorinson Midwifery Advisor, Professional Practice Unit at the Australian College of Midwives advises caution when interpreting the results, as further research is needed."This small study demonstrates that a small proportion of women may find hormonal contraception less effective, particularly low-dose formulations," she says. "However, it is important to remember that this is a small study, and the first of its kind."


Mrs Rorinson notes that often there are multiple genetic and environmental factors which determine a drug's effect on an individual. "The same drug can have drastically different effects on two people, so it is important that women talk with their health care professional about any medications they take, including side-effects and effectiveness," she continues. Adding that no method of contraception is 100 per cent effective, even when used as directed (barring abstinence) Mrs Rorinson says, "I encourage women who have concerns about the effectiveness of their contraception to discuss this with their doctor or midwife. "

According to Marie Stopes Australia contraceptive implants prevent pregnancy by releasing small but constant amounts of the hormone progestogen into the body via the bloodstream. The hormone blocks ovulation and it thickens the mucus at the cervix so that sperm cannot get through to meet an egg.

It is more than 99.9 per cent effective and lasts for three years.

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