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Body and mind

Pregnancy associated with protection against breast cancer

New Danish research shows that giving birth to children can significantly reduce women’s risk of breast cancer. However, the stage in the pregnancy at which the woman gives birth matters. If the baby is born before week 34 of the pregnancy, the woman is not protected against breast cancer, whereas a longer pregnancy protects. Giving birth to more than one child also improves the protection, and women older than 28 years who have children do not achieve protection.

Worried about breast cancer?

New Danish research suggests that having children at a young age is associated with a markedly reduced risk of breast cancer later in life.

The researchers obtained information from 2.3 million women in Denmark and 1.6 million women in Norway, and the conclusion is clear: every pregnancy reduces the risk of breast cancer later in life by 8–9%.

“It has long been known that giving birth is associated with a lifelong reduction in the risk of breast cancer. This study went a step further and can now determine how long women must be pregnant before achieving protection,” says the researcher behind the new study, Mads Melbye, Professor, Statens Serum Institut, Copenhagen, Denmark.

The research results, in which PhD student Anders Husby is the first author, were recently published in Nature Communications.

Pregnant women protected after 33 weeks of pregnancy

The researchers obtained data from various Danish registries, including cancer registries, birth registries and abortion registries. Using these data, they identified the extent to which being pregnant reduced the risk of breast cancer and when in the pregnancy the protective effect occurs.

Previously, researchers used data from the Danish abortion registry to show that being pregnant for 8–12 weeks and then getting an abortion does not protect against breast cancer. A longer pregnancy is needed before the protective effect occurs, and the researchers wanted to determine how long the pregnancy should last to provide protection.

The new results show that the protective effect only occurs after week 33 of the pregnancy. Women who give birth up until week 33 of pregnancy do not achieve protection, whereas women who give birth during week 34 of pregnancy or later obtain protection.

In addition, the results show very clearly that this effect does not increase slowly but changes very abruptly, with the cut-off between weeks 33 and 34 of pregnancy.

“The Danish registries are renowned globally because they go way back in time, are very complete and cover the entire population. We used the registries to very clearly determine when the protective effect occurs. We also found that each additional child increases the protective effect by about 8–9%. This means that the more children a woman gives birth to, the better she is protected against breast cancer for the rest of her life,” Mads Melbye explains.

Translating new knowledge into preventive treatment

Mads Melbye cannot yet explain why pregnancy and giving birth are associated with protection against breast cancer.

He says that the cells in the breasts undergo major changes in pregnancy, and towards the end of pregnancy many breast cells mature for the purpose of producing milk.

Breastfeeding itself is not the explanation, because the researchers found the same protective effect among women who experienced stillbirth.

The researchers are currently investigating the composition of several thousand substances in women’s blood in weeks 33 and 34 of pregnancy to determine whether this composition changes and might help to explain the protective effect.

Mads Melbye imagines that some substances are released into the blood at this specific time in pregnancy and help to mature and thus also protect the breast cells.

“If we can determine which substances provide this protective effect, we could clearly convert them into preventive treatment as medicine. This is the ideal scenario in the long term,” says Mads Melbye.

Protection ends after the mother reaches 28 years

The new research findings show that women should preferably be young for a pregnancy to achieve the protective effect.

Women who give birth when they are younger than 28 years old achieve the full effect of having been pregnant for more than 33 weeks, whereas a pregnancy after 28 years old does not produce any effect.

Again, the researchers face a question that they cannot yet answer, but the result is worth noting.

“It is interesting that, in Denmark, as women giving birth for the first time become older and older, the number of women with breast cancer has increased. These two trends may be linked,” according to Mads Melbye.

Identical results in Norway

To ensure that the findings do not solely apply to women in Denmark, the researchers confirmed their results by analysing the data from Norwegian databases. The results were not merely similar to those from Denmark but identical.

The women in Norway also achieved a protective effect against breast cancer if they were pregnant for more than 33 weeks.

“This confirmed our findings and shows that our results are very solid,” says Mads Melbye.

"Pregnancy duration and breast cancer risk" was published in Nature Communications.  In 2009, the Novo Nordisk Foundation awarded a 10-year grant to the Danish National Biobank at Statens Serum Institut.

Mads Melbye
Clinical Professor