In a world first, the study analysed completely healthy breast tissue, enabling insight into the development of cell mutations at the earliest stages.

Prof Stebbing noted that previous papers had to rely on tissue taken close to a cancer or, for example, from an adjacent breast in a sick patient.

“Normal tissue gave us a crucial opportunity to evaluate cellular degradation whilst factoring in age and pregnancy,” he said. “There are two types of mutations we typically see in cancer cells, and we refer to these as passengers and drivers.

“Drivers are the more dangerous ones and, surprisingly, it was these very ones that we spotted in the older pregnant women – a linear increase in the epithelial cells with age.

“We have discovered that a combination of late pregnancy and age foster a huge number of cell mutations. If you take a woman who has never had kids, the mutational burden goes up and up with age.

“However, we now know what happens is that with late pregnancy you get an enrichment on top of that, of what we call mutated clones – groups of cells with important mutations in them.”

The paper was published on an open access pre-print repository this week and will now be subject to a peer-review process.  It is the first study to demonstrate precisely how pre-cancerous mutations develop in the breast and where.

Dr Biancastella Cereser, who designed the study with Prof Stebbing, told The Telegraph: “We biopsied branches of the duct, the more functional part of the breast, but we also looked at epithelium cells, where cancer usually starts. We extracted DNA from there and also the surrounding area of tissue.

“The colon and some other organs have been sequenced but not the breast, cancer there is more complex because of the influence of hormonal changes.”  

The positive impact from the study could be twofold, Prof Stebbing said, adding: “With this kind of information, we can start to identify the specific women who need to have their breasts closely monitored during pregnancy.

“Secondly, this data brings us another step forward to a future where cancer patients will be prescribed medication specifically suited to their genes.”

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