The Who, What, Where, When and Sometimes, Why.

Research table: Birthweight and breast cancer risk

This summary table contains detailed information about research studies. Summary tables are a useful way to look at the science behind many breast cancer guidelines and recommendations. However, to get the most out of the tables, it’s important to understand some key concepts. Learn how to read a research table.

Introduction: Birthweight is your weight when you were born.

Studies show women who had a higher birthweight have an increased risk of breast cancer compared to women who had a lower birthweight. This is most clearly seen in premenopausal (before menopause) women.

Learn more about birthweight and breast cancer risk.

Learn about other early life exposures and breast cancer risk.

Learn about the strengths and weaknesses of different types of studies.

See how this risk factor compares with other risk factors for breast cancer.

Study selection criteria: Large meta-analyses.

Table note: Relative risk above 1 indicates increased risk. Relative risk below 1 indicates decreased risk.


Study Population
(number of participants)

Comparisons of Birthweight

Relative Risk of Breast Cancer in Women with a Higher Birthweight versus Women with a Lower Birthweight,
RR (95% CI)

Before Menopause

After Menopause

Before or After Menopause


Xue and Michels [1]

18 studies

Higher vs. lower




Zhou et al. [2]

16 studies

Higher vs. lower




Park et al. [3]

11 studies

6 lbs. 10 oz. – 8 lbs. 13 oz.
Less than 6 lbs. 10 oz.






More than 8 lbs. 13 oz.
Less than 6 lbs. 10 oz.




Michels and Xue [4]

12 studies

Higher vs. lower





6 studies

Higher vs. lower




* Relative risk for the 4 cohort studies was similar, 1.01 (0.79-1.31).

† Relative risk for the 4 cohort studies was 1.21 (0.80-1.82).

‡ Relative risk for the 5 cohort studies of premenopausal breast cancer was similar, 1.20 (1.07-1.35). Relative risk for the 3 cohort studies of postmenopausal breast cancer was similar, 1.04 (0.91-1.19).


  1. Xue F, Michels KB. Intrauterine factors and risk of breast cancer: a systematic review and meta-analysis of current evidence. Lancet Oncol. 8(12):1088-100, 2007.
  2. Zhou W, Chen X, Huang H, Liu S, Xie A, Lan L. Birth weight and incidence of breast cancer: dose-response meta-analysis of prospective studies. Clin Breast Cancer. 20(5):e555-e568, 2020.
  3. Park SK, Kang D, McGlynn KA, et al. Intrauterine environments and breast cancer risk: meta-analysis and systematic review. Breast Cancer Res. 10(1):R8, 2008.
  4. Michels KB, Xue F. Role of birthweight in the etiology of breast cancer. Int J Cancer. 119(9):2007-25, 2006.

Updated 11/07/23