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A genetic mutation known to increase a woman's risk of cancer could also increase their fertility, research suggests. Women with mutations in the BRCA1 or BRCA2 genes, which are associated with a higher risk of breast and ovariancancer, were found to have larger families when compared to control groups.

Professor Ken Smith and his colleagues, from the University of Utah, USA, used data on over 100,000 women in total, looking at the number of children borne by women with BRCA1/2 mutations. They were found to have a larger average number of children when compared to those without the mutations; they also had their first child earlier and their last later, with a smaller than average gap between each child.

In order to account for the effect of modern contraception, the study, published in the journal Proceedings of the Royal Society B, distinguished between women born before 1930 and those born afterwards. Increased fertility was much more pronounced in women with BRCA1/2 mutations born before 1930 – they were on average 3.6 times more likely to have four or more children than their pre-1930 counterparts without the mutations.

Usually, genetic mutations which significantly increase cancer risks and mortality are gradually eliminated by natural selection – higher mortality risks are generally associated with a lower chance of reproduction. The BRCA1/2 mutations' role in increasing women's fertility could explain why they remain relatively prevalent. 'For us the question was why are these particular genetic mutations still present in the population', said Professor Smith.

The authors wrote that 'the very individuals who carry these mutations are most likely to transmit them, given their larger family sizes'. The study also noted that 'women with BRCA1 or BRCA2 mutations have an estimated 40 to 85 percent lifetime risk of developing breast cancer and 16 to 64 percent risk of ovarian cancer'. According to the US National Cancer Institute, women with the mutations are about five times more likely to develop breast cancer.

Previous smaller studies had not found any link between BRCA1/2 mutations and fertility, as noted in Professor Smith's paper. According to the Australian Broadcasting Corporation, Professor Rodney Scott, an Australian geneticist from the University of Newcastle, said that the findings are 'certainly worth following up', but cautioned that the results could be down to cultural factors.

Professor Scott said that the researchers had only looked at married women from families in Utah with a high incidence of breast cancer. 'These families are not necessarily representative of women in the general population who carry BRCA1 or BRCA2 mutations', he explained. 'One needs to look more closely at some of these confounders to prove whether or not this is a real finding or a mysterious finding as a result of some bias'.