An unintended pregnancy while using hormonal contraception may not always be down to the woman’s mistake, according to research that suggests for some genetics could play a role.
Millions of women use hormonal contraceptives such as different types of the pill, contraceptive implants or hormone-releasing intrauterine systems (IUS) or vaginal rings. These devices release hormones to prevent the release of an egg, as well as triggering other changes in the body to prevent a pregnancy.
A small proportion of these women still become pregnant, which experts generally put down to the method not being used correctly or the woman being in the very early stages of pregnancy when she starts the contraception.
However, researchers say they have found tentative signs that there may sometimes be a different reason: some women’s bodies break down the hormones released by the contraceptive faster, potentially reducing its efficacy.
While experts say women should not be concerned about their hormonal contraception, particularly long-acting methods, they say the finding merits further scrutiny.
“We as doctors, [as] healthcare providers, have always assumed [that if] a woman comes and says ‘I got pregnant, I was using birth control,’ she must have done something wrong. And that is maybe not the case,” said Dr Aaron Lazorwitz, the first author of the study, from the University of Colorado’s school of medicine. Writing in the journal Obstetrics and Gynecology, Lazorwitz and colleagues report how they found the link by taking blood samples from 350 healthy young women, all of whom were using the contraceptive implant.
The implant is one the most effective methods of contraception and user error is virtually impossible as it sits under the skin in the upper arm. It steadily releases a progestin called etonogestrel, a hormone that is not naturally found in the body.
The team looked at whether the women had certain genetic variants that had previously been linked to the way the body breaks down or regulates female sex hormones. They then investigated whether the genetic variants found were linked to clear differences in levels of etonogestrel in the women’s blood.
The results reveal that both body mass index and the duration for which the implant was fitted affected the concentration of etonogestrel in the women’s blood.
However, after taking into account such factors, three genetic variants also slightly affected the etonogestrel levels, one of which was linked to a drop of 23% on average. This genetic variant, CYP3A7*1C, was found in 5% of the women.
While the team say the link is not robust, they found 28% of women with this genetic variant had levels of etonogestrel below the threshold that the producers of the implant say are needed to prevent the release of an egg from a woman’s ovaries. By contrast, among women without this genetic variant, only 9% had etonogestrel levels below that threshold.
The authors suggest other changes induced by the implant, such as a thickening of cervical mucus, could mean most women with CYP3A7*1C remain protected against pregnancy.
Lazorwitz said women using the contraceptive implant or IUS should not be concerned by the findings. “Despite what we found, that genetics can affect the birth control hormones, we still know that the implant and the IUS are the best birth control methods by far,” he said.
The team said it was too soon to suggest women are screened for the genetic variant. But they said the findings merit further scrutiny, particularly in relation to users of contraceptive pills as these use similar progestins to the implant but at lower levels.
Dr Olivia Fletcher, a team leader in functional genetic epidemiology and an expert in CYP3A7*1C at the Institute of Cancer Research, London, noted that the research did not analyse data separately for women of different ethnicities, which could influence the results.
Nonetheless, she said the proposed influence of the gene variant on etonogestrel levels was plausibleand consistent with previous research on its effects on female sex hormones found naturally in the body.
Prof Sharon Cameron, a co-director of the clinical effectiveness unit of the Faculty of Sexual and Reproductive Healthcare, said women should not be concerned and the contraceptive implant had a very low failure rate, with only one in 1,000 women becoming pregnant over three years of use.
“Although genetic makeup might have an impact on how well contraception might work for some women, this is only one study and it is very early to tell whether this [genetic testing] technology will be helpful in the future,” she said.