The scientific backdrop to this study is a genuine epidemiological puzzle. For decades, researchers have known that women who have given birth to more children experience a lower incidence of breast cancer after menopause, with the risk declining roughly linearly as parity increases. Pregnancy induces profound and lasting changes in breast tissue, including differentiation of the mammary epithelium, and many investigators have assumed that these changes would be reflected in altered gene expression profiles that could be detected decades later. Yet the precise molecular mechanism by which childbearing confers its protective effect has remained elusive, and attempts to pin it down at the level of tissue transcriptomics have produced inconsistent results.

The NOWAC research group, led by Eiliv Lund of UiT The Arctic University of Norway in Tromsø, had previously reported a striking observation that seemed to point toward a mechanism. In studies of peripheral blood cells from healthy postmenopausal women, the team demonstrated that increasing parity was associated with a linear decrease in the expression of hundreds of genes. This gene expression signature in blood cells appeared to mirror the linear decline in postmenopausal breast cancer incidence with increasing parity. Intriguingly, however, the same parity-related expression changes were not observed in the blood cells of women who had already developed breast cancer, suggesting that the signature might represent a protective state that is lost or overridden during carcinogenesis.

Those earlier findings raised an obvious and important question: if parity reshapes gene expression in blood cells in a way that tracks cancer risk, does it also reshape gene expression in the breast tissue itself, where tumors actually arise? Answering this question required tissue samples that are extraordinarily difficult to obtain from healthy women, because there is ordinarily no clinical reason to biopsy normal breast tissue. The NOWAC infrastructure, a prospective cohort study that enrolled 172,000 women between 1991 and 2007, provided the framework needed to attempt it.

Between 2006 and 2010, the researchers recruited women in two ways. Women enrolled in NOWAC who received a diagnostic biopsy for breast cancer were asked to provide a second biopsy specifically for research purposes, with the collaboration of surgeons at ten hospitals participating in the Norwegian Breast Cancer Group. In parallel, normal breast tissue was collected from NOWAC participants undergoing the national mammographic screening program, an effort made possible by a feasibility study showing that 74 percent of healthy women approached at a screening center in Tromsø agreed to donate tissue. The final analytical dataset consisted of 279 age-matched case-control pairs, and the paired matched design was maintained throughout all laboratory analyses, a methodological choice that strengthens the comparison between cancer patients and healthy controls.

The laboratory work relied on Illumina microarray technology to measure genome-wide gene expression, with differential expression analysis performed using the limma Bioconductor package in R, a standard and widely trusted tool for microarray statistics. To characterize the molecular subtype of each tumor and each normal tissue sample, the researchers applied the PAM50 test, a 50-gene classifier that assigns breast tumors to intrinsic subtypes such as luminal A, luminal B, HER2-enriched, and basal-like. The statistical framework controlled the false discovery rate at a q-value threshold of 0.05, ensuring that reported findings were unlikely to be artifacts of multiple testing across thousands of genes.

The results on the cancer side were dramatic, but not in the way the parity hypothesis would predict. When the researchers compared breast cancer tissue with normal breast tissue, they found that nearly the entire transcriptome was differentially expressed: 10,013 of the 11,308 genes assessed showed statistically significant differences between cases and controls. Of these, 5,768 genes were upregulated and 4,245 genes were downregulated in cancer tissue, a sweeping molecular signature of the malignant state that confirms the biological gulf between tumor and normal tissue. This massive case-control signal also demonstrates that the study had ample statistical power to detect expression differences, which makes the central null finding all the more meaningful.

That central finding is the study’s headline result. When parity, classified into three categories of zero children, one to three children, and four to eight children, was incorporated into the statistical model as an explanatory variable, only three genes in breast cancer tissue showed significant changes associated with parity, and not a single gene in normal breast tissue did. Gene set enrichment analyses, which test whether predefined biological pathways are collectively altered, returned non-informative results. Furthermore, the PAM50 classification failed to establish any relationship between parity and the distribution of intrinsic molecular subtypes in either tissue type. In other words, across the entire measurable transcriptome, the number of full-term pregnancies left virtually no trace in the breast.

The authors conclude that their study revealed no significant differences in gene expression between normal and breast cancer tissue in relation to the number of full-term pregnancies. This is a genuine null result from a well-powered, carefully matched design, and null results of this kind carry real scientific weight. They effectively rule out a whole class of mechanistic explanations: whatever protects parous women from postmenopausal breast cancer, it does not appear to operate through durable, detectable changes in the steady-state expression levels of genes in breast tissue, whether healthy or malignant. The protective signal that the team previously observed in peripheral blood cells now stands in sharper contrast, suggesting that the parity-related biology may be systemic rather than resident in the breast itself, or that it operates through mechanisms invisible to bulk transcriptomics, such as epigenetic marks, stromal architecture, immune surveillance, or transient changes that fade long before menopause.

The study also stands as a testament to the value of long-term prospective cohorts with biobanks. Only a resource like NOWAC, which links decades of lifestyle and reproductive history data to consented tissue donations and national cancer registry records, could assemble 279 matched pairs of cancer and normal breast tissue with complete parity information. The work was supported by the European Research Council, the Northern Norway Regional Health Authority, and the Norwegian Cancer Society, and the authors emphasize that the funders played no role in the design, analysis, or interpretation of the study. For researchers hunting the molecular basis of pregnancy’s protective effect, the message is clear: the answer will not be found by simply cataloging which genes are switched on or off in breast tissue. The search must now move to other layers of biology, and this study, by closing one door definitively, helps point the way.

Subject of Research: The effect of parity on global gene expression in normal breast tissue and breast cancer tissue

Article Title: No impact of parity on global gene expression levels in breast cancer tissue and normal breast tissue – a nested case-control study in the Norwegian Women and Cancer study

Article References: Lund, E., Krum-Hansen, S., Olsen, K. S., Shvetsov, N., Snapkow, I., Gavriluk, O., Busund, L.-T. R., Frantzen, J. O., Holden, M., & Holden, L. (2026). No impact of parity on global gene expression levels in breast cancer tissue and normal breast tissue – a nested case-control study in the Norwegian Women and Cancer study. BMC Cancer. https://doi.org/10.1186/s12885-026-17044-5

Image Credits: AI Generated

DOI: 10.1186/s12885-026-17044-5

Keywords: breast cancer, parity, gene expression, NOWAC study, transcriptomics, microarray, PAM50, nested case-control, epidemiology, reproductive history, cancer prevention, Norway