Early first birth may offset breast cancer risk tied to early periods in UK Biobank study

Women who started menstruating early may see their elevated breast cancer risk tempered if they gave birth to their first child by age 30, according to an analysis of more than 250,000 women in the UK Biobank cohort published in Breast Cancer Research. The study, based on data from 254,578 women, found that age at first live birth modified the well-known association between early menarche and breast cancer incidence.

Over a median follow-up of 13.9 years, 11,133 breast cancer cases occurred in the cohort. The researchers, using Cox proportional hazards models, confirmed that each one-year delay in the age at which periods began was associated with a 2% lower breast cancer incidence rate, with a hazard ratio of 0.98 (95% confidence interval 0.97–0.99). Early menarche has long been recognised as an established risk factor, but the authors set out to test whether later-life reproductive events and exogenous hormonal use could modify that elevated risk.

Among women whose periods began at age 11 or earlier, a first live birth at or before age 30 showed a statistically significant interaction on the multiplicative scale (likelihood ratio P = 0.002) and a negative interaction on the additive scale, with a relative excess risk due to interaction of −0.28 (95% CI −0.48 to −0.09). In stratified analyses, an early first birth was associated with a 28% lower relative rate of breast cancer compared with a first birth after age 30 among women with early menarche (adjusted hazard ratio 0.72; 95% CI 0.63–0.84).

The absolute risk difference was modest but measurable. Model-based estimates for the same subgroup indicated an approximately 0.9 percentage-point lower 10-year absolute breast cancer risk at age 60 years for women with early menarche who had their first live birth by 30, compared with those who did not. No other later-life factors examined in the study showed significant effect modification of the early menarche association.

The analysis also explored possible biological mechanisms. Using multivariable linear regression, the researchers compared circulating hormone profiles across strata defined by age at first live birth. Among women with early menarche, those with a first live birth at or before age 30 exhibited circulating hormone profiles that have previously been reported to be associated with lower breast cancer risk, offering a tentative physiological explanation for the observed interaction.

The authors conclude that an earlier age at first live birth may attenuate the increased breast cancer risk associated with early menarche. Because the findings come from an observational cohort analysis, they describe associations rather than demonstrated causal effects, and the reported hormone-profile findings reflect patterns previously linked to risk rather than proven protective mechanisms. The article was published on 15 October 2026 as an open-access research paper.

Subject of Research: Effect modification of the early menarche and breast cancer association by age at first live birth in the UK Biobank cohort

Article Title: Age at first live birth modifies the association between age at menarche and breast cancer risk in the UK Biobank cohort

Article References: Li, Y., Yang, X., Guo, S., Huang, J., Ren, X., Zhou, Q., Zhu, X., Zhang, Y., Hou, H., Zhao, Y., & Hua, B. (2026). Age at first live birth modifies the association between age at menarche and breast cancer risk in the UK Biobank cohort. Breast Cancer Research. https://doi.org/10.1186/s13058-026-02403-8

Image Credits: AI Generated

DOI: 10.1186/s13058-026-02403-8

Keywords: breast cancer, menarche, age at first live birth, UK Biobank, cohort study, hormone profiles, reproductive factors, effect modification, absolute risk, epidemiology, first, live