Landmark study confirms that having children naturally protects against breast cancer – and experts say this could be why cases are rising in under 50s

It has long been known that women who have children are at a reduced risk of breast cancer, but until now experts didn’t understand why. 

Now Australian researchers have shown that the natural protection against breast cancer offered by childbearing could be linked to an influx of killer cells in the breast. 

The team – from Peter MacCallum Cancer Centre – hope that doctors will soon be able to induce this natural protection without the need to conceive, offering hope to women who can’t or don’t want to have children. 

‘Our research shows that women had better outcomes if they have breast tissue with high levels of Killer T-cells – a specialised immune cell that helps detect and destroy abnormal cells like cancer,’ Professor Kara Britt, the study’s co-lead author, said. 

‘We uncovered that these T cells are recruited during pregnancy, but survive for a long time and potentially stay in the breast tissue patrolling it for abnormal cells for years,’ she added.

These protective cells were found in breast tissue up to ten years post pregnancy. 

The findings come following a sharp rise in the number of young people diagnosed with the disease, with 8,500 cases among young women every year in the UK.

It has long been suggested that having children earlier in life can help protect against ovarian and breast cancer. But experts have cautioned against raising fears among career women who delay having children, and those who can‘t conceive.

The study – published in the journal Nature Immunology – found that pregnancy hormones trigger the production of these killer cells in the breast, around halfway through pregnancy. 

These cells then remain well after breastfeeding, for up to 10 years, exerting a long-term anti-tumour effect and providing protection against breast cancer. 

Models suggest that T-cells – located within the milk ducts – rely on milk-producing breast cells for survival and growth, offering an explanation as to why only women who have a full-term pregnancy experience a reduced risk 

Using hormone treatments, researchers were able to bring about the same changes in breast cells to trigger this influx of T-cells that protect against breast cancer – without the need for pregnancy.

The team concluded: ‘These findings are very exciting and open up new avenues for immune-preventative interventions that may reduce the development of breast cancer in high-risk populations who have not conceived.’ 

While the link between reproductive factors, hormones and a woman’s cancer risk is extremely complex, separate research has suggested having children younger can protect against the disease.  

This is because breast cells remain in an immature and vulnerable state until a woman gets pregnant. 

These immature cells are more sensitive to oestrogen and other growth-stimulating hormones and are more likely to grow in abnormal ways, increasing the risk of cancer. 

Having children earlier means these cells – there to produce milk – are able to fulfil their natural function sooner, reducing the window of time for cells to multiply. 

And Dr Andrea DeCensi, director of medical oncology at Galliera Hospital in Italy, believes this could help explain the surge in cases in under 50s – as women are getting pregnant later in life, or not at all. 

Speaking at the annual meeting of the American Society of Clinical Oncology, he said: ‘People are reluctant to talk about it, but delayed childbearing is one of the biggest contributors to rising breast cancer rates.

‘Biologically, women are ready for pregnancy soon after their first period.

‘The optimal window for childbearing is between 20 and 35. After that, not only does it become more difficult to conceive, but breast cancer risk significantly rises. And this is something a lot of women don’t know.’

According to a study published in the British Journal of Cancer, women who have their first child in their thirties are more than 60 per cent more likely to develop the disease before the menopause, than those who give birth at 22.

And the risk falls by up to nine per cent with each pregnancy.

Breastfeeding has also been shown to have a protective effect, delaying the onset of the disease by ten years – but only in women who do so for over six months and don’t smoke, research suggests.

Commenting on the study, Simon Vincent, chief scientific officer at Breast Cancer Now, said: ‘We know from the Breast Cancer Now Generations study that pregnancy and childbirth reduce breast cancer risk in the long term. But we don’t know why.

‘This study offers early evidence that the changes to the immune system in the breast during pregnancy may explain that long-term protection from breast cancer. We now need more research to understand this more and to see if it can open the door to new ways to prevent breast cancer.

‘It’s very important to note that whether and when to have children is a very personal decision. And while we know that factors like mother’s age at first childbirth, the number of children they give birth to and breastfeeding can have an effect on breast cancer risk, it is also influenced by genetics and many factors outside of our control.’

Breast cancer is just one of the 11 cancers on the rise among young people, but no single cause has been identified. 

It is now the most common cancer in the UK, with more than 59,000 new cases every year. But survival rates are high with around 77 per cent of women living with the disease for ten years or more.