Pregnancy duration influences a woman’s risk of breast cancer

Giving birth reduces a woman’s risk of developing breast cancer later in life. Danish researchers from Statens Serum Institut (SSI) report that the protective effect arises in a particular week of pregnancy

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It has long been known that giving birth, preferably more than once, early in life reduces a woman’s risk of developing breast cancer later.

In a large new study from Statens Serum Institut published in the scientific journal Nature Communications, researchers report that for each child born to a woman before the age of 28 years, her risk of developing breast cancer later in life is reduced by approximately 8 percent. However, births after the age of 28 years do not contribute further protection.

The role of age per se is interesting. But the primary purpose of the study was to unravel what makes a pregnancy provide life-long protection against breast cancer.

Researchers from Statens Serum Institut have taken an important step toward answering this question. The protective effect of pregnancy was not observed in women who gave birth within the first 33 weeks of pregnancy. In contrast, women with a pregnancy duration of 34 weeks or more had a reduction in their life-long risk of developing breast cancer.

All Danish and Norwegian women were studied

‘It is a very important to know that the protective effect arises at a particular time in a woman’s pregnancy rather than gradually occurring throughout the pregnancy. Now, researchers know where to look for the protective factor, making it possible to potentiallyuncover the mechanism and understand how the protection is achieved’, says Professor Mads Melbye from SSI, principal investigator of the study.

The study was performed by researchers at SSI, with PhD student Anders Husby as first author. The study was carried out in collaboration with Professor Nina Øyen from the University of Bergen, Norway, and the results have just been published in the international journal Nature Communications.

The sensational findings are based on an extremely large number of women. The researchers studied all Danish women born between 1935 and 2002 – a total of 2.3 million women – and followed their pregnancy histories and breast cancer outcomes in the period 1978-2014. The total study base amounted to around 3.3 million pregnancies.

The Danish numbers were then compared with an equivalent study of all Norwegian women – a total of 1.6 million women – born between 1935 and 1994.

A pill against breast cancer?

The results from Denmark and Norway were identical. Moreover, the study showed that the protective effect was independent of a woman’s socio-economic status and of whether the child was live born or stillborn at birth. Only the pregnancy duration was significant.

‘Generally speaking, giving birth to a child in your 20s is the strongest known protective factor in terms of breast cancer risk. We have now shown that the protective effect arises exactly in week 34. Prior to this week, pregnancy appears not to offer any protection against breast cancer’.

It seems reasonable to hypothesise that the protective effect is associated with signals initiating maturation of breast tissue to allow them to produce milk in time for the coming infant’, says Mads Melbye, who hopes that this new study may contribute significantly to the fight against breast cancer.

‘We can learn a lot about how to reduce a woman’s risk of developing breast cancer. It we are able to identify which molecular features in the body that initiate the protective effect, we might eventually be successful in developing a pill to protect women against breast cancer. This has long been my dream’, concludes Melbye.

Breast cancer is the most common cancer among women. Approximately every 9th woman develops breast cancer, and each year 4,000 Danish women develop the disease.

Read the article in Nature Communications here