Study: Breast cancer risk increases modestly after childbirth

IN-DEPTH REVIEW OF RESEARCH

Study background:

In the United States, breast cancer is diagnosed in 12% of women (1 in 8) in the general population. While the average age at breast cancer diagnosis is 60 years, a significant proportion of women are diagnosed at younger ages.

According to a recent SEER report, for a woman in the general population, the risk of being diagnosed with breast cancer in the next ten years is:

• At age 30: 0.44% (1 in 227)
• At age 40: 1.47% (1 in 68)
• At age 50: 2.38 % (1 in 42)

This risk is higher for women with mutations in breast cancer predisposing genes (e.g., BRCA1, BRCA2, PALB2, etc.); on average, these women tend to be diagnosed when younger, when they are often still in the midst of family planning.

Over a woman's lifetime, childbearing is associated with a decreased risk of breast cancer in women, as compared to women of the same age who never bear a child. Several studies have indicated that there may be an increase in breast cancer risk immediately after childbirth, and/or with a woman's increased age at first childbirth.

To clarify the relationship between childbearing and breast cancer risk, researchers in this study evaluated 15 prospective studies that followed women from enrollment before a breast cancer diagnosis. They collected information on births, breastfeeding, breast cancer and tumor type, age of mother at childbirth, and time elapsed since childbirth.

Researchers of this study wanted to know:

The breast cancer risk associated with childbearing.

Populations looked at in this study:

This study is a meta-analysis by the Premenopausal Breast Cancer Collective that pools data from 15 prospective studies to examine features that might affect breast cancer risk after childbearing. Participants were women under 55 years old who did not have breast cancer at the time of enrollment. Women were surveyed or contacted for follow-up until a breast cancer diagnosis, age 55, last scheduled study follow-up or death, whichever occurred first. The time since last birth was reset with additional births during the follow-up time period.

Study findings: 

Among 889,944 women ages 55 and younger, 18,826 cases (2.1%) of breast cancer were diagnosed. Among participants, 720,555 women (81%) had given birth to a child at the time of enrollment and 71,609 (8%) had 1 or more children during the follow-up time period.

The factors that altered breast cancer risk after childbirth included:

• number of children
• family history
• age of first birth

Notably, breastfeeding did not contribute to breast cancer risk in this meta-analysis of 15 studies. Past reports suggested that breastfeeding may lower risk for ER-negative breast cancer and that the risk for breastfeeding women was similar to the risk of women who never gave birth. However, in this study, no significant increase or decrease in breast cancer risk with breastfeeding was found.

Women had a modest but higher risk of breast cancer after childbirth—for up to 20 years—than women who did not give birth.

• Risk peaked at 5 years after birth with a relative risk of 1.8 (an 80% increase in breast cancer risk for women who had childbirth versus women who did not). (For this evaluation, the control group consisted of women who had never delivered children and who were designated as having a relative risk of 1.0.)
• Risk declined over time until 24 years after childbirth.
• By 24 to 34 years after childbirth, women who had children had decreased breast cancer risk versus women who had never had children. The hazard ratio at 34 years after birth was 0.77, a 23% decrease in risk among women who had a child versus women who never had children.

Childbearing increases breast cancer risk significantly, but modestly.

How does this relative risk translate into absolute risk? (Relative risk reflects how much something you do increases or decreases your risk relative to a comparison group. Absolute risk is the likelihood of developing cancer over a given period.)

Comparing women whose most recent child was born 3-7 years prior, breast cancer occurrence per 100,000 women was:

• by age 45:
  • 620 women who had never given birth    
  • 661 women who had given birth
• by age 47.5:
  • 1,252 women who had never given birth
  • 1,422 women who had given birth
• by age 50:
  • 1,955 women who had never given birth
  • 2,202 women who had given birth

A woman’s age at first childbirth elevates breast cancer risk.

Breast cancer risk was greater for women who had their first child later in life. The older a woman was when she has her first child, the more likely she was to have breast cancer in the 5 years after giving birth.

Women who had their first child when they were younger than 25 had no increased risk compared to women who never had children.

How many children a woman has impacts her breast cancer risk.

The modestly increased risk of breast cancer seen with childbirth was observed with the birth of subsequent children.

• Breast cancer risk was greater for women who had 3 or more children; the period of time before that risk abated was longer than for women who had 0, 1 or 2 children.

Breast cancer risk is greater for women with a family history of breast cancer.

Women with a family history of breast cancer also have increased risk after childbearing. This risk appears to be additive. The additional increase in risk of breast cancer after childbirth among women with a family history of cancer is a similar to the amount of additional increase in risk of breast cancer after childbirth among women without a family history of cancer.

Breast cancer risk varies by tumor type.

Among participants, 76% of breast cancer cases were ER-positive tumors and 24% were ER-negative tumors.

• Among women with ER-positive tumors, the risks of breast cancer after childbirth paralleled the overall study results detailed above: a modest increase in risk of having an ER-positive tumor after childbirth was observed.
• Among women with ER-negative tumors, the risks of breast cancer after childbirth were slightly lower (HR=1.7 ER-negative vs HR 1.8 overall), but declined more slowly with time.
• Breastfeeding (or not) had minimal orto no impact on ER-positive or ER-negative tumors.

Limitations:

This meta-study combines data from multiple original studies. Relative risk was determined by comparing women with children to women without children, yet the risk of breast cancer among women without children may also be changing over time.

The data available was limited to year of cancer diagnosis or birth, so researchers were unable to distinguish births that occurred during pregnancy from those occurring soon after birth.

Breast cancer risk after age 55 was not evaluated.

Intervals between births (either longer or shorter) were not evaluated. Time since childbirth was reset to zero with each additional birth.

Among women with multiple children, breastfeeding status could have been incorrect for a woman’s most recent birth if she breastfed one/some but not other children.

While this study addresses the difference in risk of ER-positive versus ER-negative breast cancer after childbirth, the information about tumor subtypes was limited. Because data on HER2 status was not available for all participants, the extent to which HER2-positive or HER2-negative breast cancer occurs after childbearing remains an open question.

The genetic status of participants was unknown. Given that women who were diagnosed with breast cancer before age 40 more often have inherited mutations, a large subset of the participants with breast cancer may have an underlying mutation. 

Conclusions:

Breast cancer risk increases immediately after childbearing. This contradicts the idea that childbearing is protective (reduces) breast cancer risk. However, these statements reflect different timeframes. Having a child at some point in a woman's life reduces her lifetime risk of breast cancer. Having a child increases breast cancer risk for up to 20 years, but eventually that risk declines and is eventually lower than women who have never had a child. The immediate modest increased risk of breast cancer after childbirth is eventually outweighed by the decreased risk of breast cancer associated with having a child at some time during a woman's life.

Posted 12/28/18

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