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Reproductive factors

Reproductive factors can have an important effect on a woman’s risk for breast cancer by influencing her hormone levels, which, over a woman’s lifetime, can be associated with increased or decreased risk of breast cancer. 

Modifiable reproductive factors associated with risk of breast cancer include whether a woman has breastfed children. 

Read more below to find out changes you can make to reduce your risk of breast cancer, and what you can do.

Breastfeeding

Probable

The factor is likely to be associated with increased or decreased risk of breast cancer, but the evidence is not as strong as for convincing.

Breastfeeding is probably associated with a decreased risk of breast cancer.

The longer a woman breastfeeds, the lower her risk of breast cancer. The risk of breast cancer decreases by about 2% for every 5 months that a woman breastfeeds. This is equivalent to a 5% decrease in risk of breast cancer for a lifetime total of 12 months of breastfeeding.

When a woman is lactating (producing milk) and breastfeeding, the level of the hormone oestrogen in her body is lower. Oestrogen promotes the growth of some types of breast cancer. A period of breastfeeding reduces the amount of oestrogen that a woman is exposed to during her lifetime. This might be one of the ways in which breastfeeding protects against breast cancer.

Another possibility is that breastfeeding changes the cells in the breast in ways that make them more resistant to cancer. In addition, cells are lost from the lining of the breast ducts during breastfeeding, which might eliminate cells that have already accumulated DNA damage and that could lead to cancer in the future.

Evidence classification: Probable

Breasfeeding is probably associated with a small decreased risk of breast cancer.1

There is evidence of a dose-response relationship, that is, the longer the duration of breastfeeding, the larger the protective effect. Breastfeeding probably decreases the risk of breast cancer1, with evidence of a dose-response relationship present. The risk of breast cancer has been estimated to decrease by 2% for every 5 months that a woman breastfeeds (RR 0.98, 95% CI 0.97–0.99).1 This is equivalent to a 5% decrease in risk of breast cancer for a cumulative total of 12 months and 9% decrease in risk of breast cancer for a cumulative total of 24 months of breastfeeding.

Mechanisms

Several mechanisms have been suggested for the way in which breastfeeding might influence breast cancer risk. The most plausible is the effect of lactation-induced amenorrhea in reducing a woman’s lifetime exposure to oestrogen.2 Lactation may also cause epigenetic changes that affect the risk of cancer. Exfoliation of breast tissue during lactation may reduce breast cancer risk by eliminating cells with DNA damage.1,2

Evidence 

The WCRF/AICR concluded that ‘lactation probably protects against breast cancer’.2 This was based on analysis of 13 prospective cohort studies, which showed a small but significant dose–response effect. The risk of breast cancer was estimated to be decreased by 2% per 5-month duration of breastfeeding (relative risk [RR] 0.98, 95% confidence interval [CI] 0.97–0.99).

Several of the recent meta-analyses included in the analysis by WCRF/AICR3-5 reported a protective effect of ever breastfeeding compared with never breastfeeding.3-5 A greater protective effect of longer duration of breastfeeding was also found in two of these meta-analyses.3,6  For example, the decreased risk among women who breastfed for the longest versus shortest duration was estimated to be 0.47 (95% CI 0.37–0.60).3 Preliminary evidence indicates that the protective effect may be significant only for certain types of breast cancer, including triple negative subtypes.1,4-6

Read the full Review of the Evidence

References
  1. World Cancer Research Fund (2017). Continuous Update Project Systematic Literature Review: The associations between food, nutrition and physical activity and the risk of breast cancer. London, UK.
  2. World Cancer Research Fund International/American Institute for Cancer Research (2018). Continuous Update Project Expert Report 2018. Diet, nutrition, physical activity, and cancer: a global perspective. London, UK.
  3. Zhou Y, Chen J, Li Q, et al. (2015). Association between breastfeeding and breast cancer risk: evidence from a meta-analysisBreastfeeding Medicine 10:175–182.
  4. Islami F, Liu Y, Zhou J, et al. (2015). Breastfeeding and breast cancer risk by receptor status: a systematic review and meta-analysis. Annals of Oncology 26:2398–2407.
  5. Lambertini M, Santoro L, Del Mastro L, et al. (2016). Reproductive behaviors and risk of developing breast cancer according to tumor subtype: a systematic review and meta-analysis of epidemiological studies. Cancer Treatment Reviews 49:65–76.
  6. Ma H, Ursin G, Xu X, et al. (2017). Reproductive factors and the risk of triple-negative breast cancer in white women and African–American women: a pooled analysisBreast Cancer Research 19:6.



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