Headlines today are pretty scary for the estimated one third of 16-49 year old women in the UK who use hormonal contraception, warning that doing so could ‘double the risk of brain tumour’. But these tumours are so rare that a doubling in risk actually only equates to one extra case per 50,000 women taking the contraception per year.
Hormonal contraceptives are very widely used in the UK, and provide a near-perfect method of birth control. A story that suggests they are harmful could lead women away from using them, potentially stopping any form of birth control at all, and increasing their risk of unwanted pregnancies and the health risks that can go along with that.
The researchers who conducted this study used a case-control design. They had access to data from the entire population of Denmark, and used the National Cancer registry to find instances of glioma brain tumour in women aged 15 to 49, between 2000 and 2009. In this sample, they found 317 cases of brain tumour. This roughly equates to 5 women per 100,000, per year (by comparison, breast cancer affects 125 UK women per 100,000).
Some of these people will use hormonal contraception, and some won’t. In fact, of the 317 people with glioma, and the 2126 controls used as a comparison group, just over half used hormonal contraception.
Controls are the group of people without brain tumours who are needed to compare to those who did get tumours, to see whether contraceptive use differs between the groups. How these are selected is hugely important, as it’s easy to introduce bias in to the design if this group are different to the group of cases. These controls were randomly sampled from the Danish female population, and were matched on age, to the nearest year, with 8 controls being matched to every case.
Using these data, the authors of the study found that women who developed gliomas were more likely to have used hormonal contraception than those who did not, after taking in to account years of schooling, and history of allergy or asthma.
Does this mean hormonal contraception caused the increased risk? It’s not possible to tell from these findings, as the authors of the study make plain in their paper. Whenever you observe groups of people, rather than randomly split them to either receive the intervention or not, you can never be completely sure that there aren’t other differences between the groups that could be causing the association seen. For example, obesity is thought to impact on glioma rates, but data on this was not available for the women included in this study.
Aside from the findings in this paper, there is also some evidence that hormonal contraception could be protective against ovarian cancer, cancer of the womb, and bowel cancer, all of which are more common than these brain tumours.
So while this paper suggests that hormonal contraception use might increase the risk of these specific and rare tumours, it’s certainly not definitive proof, and even if it was, women shouldn’t make decisions about their contraception based solely on this paper.
guardian.co.uk © Guardian News & Media Limited 2010
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