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Does pregnancy raise the risk of breast cancer?

A study published in the Annals of Internal Medicine recently determined whether pregnancy affects women’s risk of breast cancer.

Breast cancer is the leading diagnosis of cancer among reproductive women.

It starts as the cancerous cells of the breast replicate to form a malignant tumor that eventually metastasizes to other parts of the body.

Over the years, there has been a belief that women who reproduce have a decreased risk of breast cancer, with pregnancy providing immunity to the female body.

However, recent studies are now indicating that this may be false, and there may be more factors that can influence the risk of breast cancer development.

Nichols and her team of researchers sought to identify the relationship between breast cancer and women’s age when they had their first child. Data was collected from 15 cohort studies involved with the international Premenopausal Breast Cancer Collaborative Group.

The study targeted women younger than 55 years old who did not have breast cancer at the time of enrolment and continued to follow these women through either direct contact or linkages with cancer registries.

They published their results in Annals of Internal Medicine. After examining over 10 years of collected data from over nine million reproductive women, the researchers calculated hazard ratios (HR) for each age of reproduction to indicate their risk of breast cancer development.

Reproduction may spike the risk of developing breast cancer

The results showed that the HR peaked in women 4.6 years after giving birth, maintained for at least 20 years, and only lowered roughly 34.5 years after giving birth.

Rather than protecting against breast cancer, reproduction spikes the risk of developing breast cancer and does not protect much later. Nichols considered this relationship to be due to the increased proliferation of breast cells during pregnancy.

The proliferation would promote the development of any previously latent tumor cells.

Furthermore, the postpartum breast microenvironment encourages breast cancer cell migration and metastases through the lactation involution or the removal of inutile milk-producing epithelial cells.

The data also showed that women who had their first birth at an older age had a higher HR than those who conceived at a younger age.

There is believed to be a higher proportion of initiated cells at an older age, hence the increased risk.

Risk of breast cancer affected by estrogen receptor expression

Other factors, such as estrogen-receptor (ER) expression, parity, and familial history of breast cancer were also found to influence the risk of development.

Interestingly, breastfeeding did not show to have a significant increasing influence on development. It even showed a 12% to 25% lower risk than premenopausal women overall.

The type of breast cancer developed also varied between the age of first birth. ER-positive breast cancer was inversely correlated, with the risk of development decreasing as the time after birth increased.

On the other hand, the risk of ER-negative breast cancer has been shown to maintain for at least 25 years after giving birth. However, breastfeeding showed the potential to mitigate any further increases in risk for ER-negative breast cancer.

Lastly, reproductive women with a familial history of breast cancer showed a 3.5 higher risk than women who were not reproductive or had no familial history.

It should be noted that a limitation of the study was breast cancer diagnoses during pregnancy not being uniformly distinguishable from early postpartum diagnoses.

Data on breastfeeding was also only available in 12 of the 15 studies.

Nonetheless, as Nichols stated in the press release this study’s findings can be used to develop better breast cancer risk models to help inform screening decisions and prevention strategies.

Though more research needs to be done, Nichols and her team challenge the misconception that pregnancy shields women from the risk of developing breast cancer and opens the potential for future innovations against breast cancer.

Written by Stephanie C. Tsang


  1. Breast cancer statistics. (2018, September 12). Retrieved from
  2. Christine J Watson. (2006, April 10). Key stages in mammary gland development – Involution: Apoptosis and tissue remodeling that convert the mammary gland from a milk factory to a quiescent organ. Retrieved from
  3. Nichols, H. B., Schoemaker, M. J., Cai, J., Xu, J., Wright, L. B., Brook, M. N., . . . Sandler, D. P. (2019, January 01). Breast Cancer Risk After Recent Childbirth: A Pooled Analysis of 15 Prospective Studies. Retrieved from
  4. Oleniacz, L. (2018, December 10). Study finds a higher risk of breast cancer for women after giving birth. Retrieved from


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