How does pregnancy affect breast cancer risk?

Pregnancy at a younger age is associated with a lower risk of developing breast cancer later in life, although it can temporarily increase risk immediately after childbirth.

What did the new study in mice find?

The study found unusual hybrid cells in the mammary tissue of older mice that had never been pregnant, suggesting pregnancy may slow the accumulation of these cells.

Can the findings in mice be applied to humans?

Experts caution that findings in mice do not necessarily apply to humans, and further research is needed to confirm these results in humans.

Why Could Having a Baby in Your 20s Lower Breast Cancer Risk? Scientists Find New Clues

Q: Do these findings mean women should have children at a certain age to prevent breast cancer?

No, the study does not suggest that women should have children at a particular age to prevent breast cancer, as many factors influence cancer risk.

For decades, scientists have known that women who have their first full-term pregnancy at a younger age tend to have a lower risk of developing breast cancer later in life. Now, researchers may have found a clue to why.

A study in mice has identified unusual breast cells that appear to accumulate with age in animals that have never been pregnant. Pregnancy seemed to slow this build-up, suggesting that changes triggered by pregnancy could influence how breast tissue ages. However, the findings do not yet show that these cells cause breast cancer in humans.

What Does Pregnancy Have to Do With Breast Cancer Risk?

The relationship between pregnancy and breast cancer is complex.

According to the US National Cancer Institute, women who have their first full-term pregnancy before age 20 have about half the breast cancer risk of women whose first full-term pregnancy occurs after age 30. The protective association is mainly seen with hormone receptor-positive breast cancer.

However, pregnancy can temporarily increase breast cancer risk in the years immediately following childbirth before the longer-term reduction becomes apparent. Researchers believe hormonal changes and rapid growth of breast cells during pregnancy may partly explain this pattern.

What Did the New Study Find?

Researchers compared mammary tissue from female mice that had been pregnant with tissue from mice that had never been pregnant. The animals were examined at different ages, including mice up to 18 months old, roughly equivalent to 55–65 years in humans.

In older mice that had never been pregnant, the researchers found unusual cells with characteristics of two different types of mammary cells. These 'hybrid' cells also produced an inflammatory molecule called interleukin-33, or IL-33.

When researchers exposed mammary cells from young mice to IL-33, the cells developed age-related changes and similar hybrid characteristics.

The findings raise the possibility that pregnancy could somehow prevent or slow the accumulation of these cells, potentially altering the way breast tissue ages.

Could These Cells Cause Cancer?

That remains unknown. The researchers suspect the hybrid cells could contribute to cancer development because changes in cell identity have been linked to cancer progression. But the study did not establish that these cells actually cause breast cancer.

The researchers are now investigating whether the cells can directly contribute to disease. Experts also caution that findings in mice do not necessarily apply to humans.

What Does This Mean for Women?

The study does not mean women should have children at a particular age to prevent breast cancer. Pregnancy is only one factor among many that influence an individual's cancer risk.

Age, family history, inherited genetic changes, reproductive history, alcohol consumption, and physical activity can all play a role. The US National Cancer Institute estimates that women have about a one-in-eight lifetime risk of developing breast cancer, although individual risk varies considerably.

For now, the new research offers another piece of the puzzle surrounding pregnancy and breast cancer. If the findings are eventually confirmed in humans, understanding how pregnancy changes breast tissue could potentially point scientists towards new ways of preventing breast cancer without pregnancy itself being the intervention.