UC Santa Cruz study finds link between pregnancy and reduced breast cancer risk

Research suggests early pregnancy can permanently change how breast cells age, preventing accumulation of cells that may contribute to tumor growth.

Published on Mar. 2, 2026

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A recent UC Santa Cruz study, published in Nature Communications, uncovered clues about the relationship between early pregnancy and reduced breast cancer risk. The research found that early pregnancy, between ages 20-30, can permanently change how breast cells age, preventing the accumulation of a type of cell that may contribute to tumor growth. The study compared mammary cells from mice that had undergone early pregnancies to those that had not, and discovered a third population of 'hybrid' cells in the non-pregnant mice that produced a signaling molecule linked to cell growth and division.

Why it matters

This study provides important insights into a long-standing mystery around why early pregnancy can reduce breast cancer risk later in life. If the hybrid cells discovered do contribute to tumor formation, this could explain the protective effect of early pregnancy. The findings open the door to future research on mechanisms underlying this cancer risk and potential new preventative therapies.

The details

The research team, led by UC Santa Cruz assistant professor Shaheen Sikandar and graduate student Andrew Olander, conducted an initial study on two groups of mice - one that underwent pregnancies between 3-6 months of age (equivalent to 20-30 in humans), and another that never became pregnant. After letting the mice age naturally, the team analyzed the mammary cells using single-cell RNA sequencing. They unexpectedly found a third population of 'hybrid' cells in the non-pregnant mice that expressed a signaling molecule called Interleukin 33, which caused healthy mammary cells to behave more like aged cells. Further experiments suggested these hybrid cells could potentially contribute to tumor formation.

  • The UC Santa Cruz study was published in Nature Communications in January 2026.
  • The initial mouse experiments were conducted in late 2020 after Sikandar set up her lab at UCSC.

The players

Shaheen Sikandar

An assistant professor of molecular, cell and developmental biology at UC Santa Cruz who led the research team.

Andrew Olander

A graduate student in Sikandar's lab who was the lead author of the study published in Nature Communications.

Carman Man-Chung Li

A professor of cancer biology at the University of Pennsylvania who found the study's findings on the signaling molecule Interleukin 33 to be significant.

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What they’re saying

“This exciting finding opens the door to future research on the mechanisms underlying this cancer risk and new therapeutic approaches.”

— Carman Man-Chung Li, Professor of cancer biology (Email to Santa Cruz Sentinel)

“We were kind of surprised. We didn't expect such a strong phenotype.”

— Shaheen Sikandar, Assistant professor (Santa Cruz Sentinel)

“This is a huge variable, but it's really important ... a large part of our population has undergone pregnancies.”

— Andrew Olander, Graduate student (Santa Cruz Sentinel)

What’s next

Sikandar and Olander are now working to answer two crucial questions: Do the hybrid cells lead to cancer formation? And is there a way to stop those cells from accumulating in the first place? Their lab has recently received funding to do more experiments and try to answer those questions, which could potentially lead to new preventative therapies for breast cancer.

The takeaway

This study provides important insights into why early pregnancy can reduce breast cancer risk later in life, suggesting it may permanently change how breast cells age and prevent the accumulation of a cell type that could contribute to tumor growth. The findings open the door to future research on new ways to prevent breast cancer, which affects 1 in 8 women.