Breast Cancer Protection From Childbirth Starts Later Than Thought

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In general, women who haven’t had a full-term pregnancy or have their first child after age 30 have a higher risk of breast cancer compared to women who gave birth before age 30. Still, a study suggests that breast cancer risk reduction from pregnancy doesn’t kick in until about 20 years after a woman’s last pregnancy. The results also found that women younger than 55 who had a full-term pregnancy had a slightly higher risk of developing breast cancer during the study.

The research was published online on Dec. 11, 2018, by the Annals of Internal Medicine. Read the abstract of “Breast Cancer Risk After Recent Childbirth: A Pooled Analysis of 15 Prospective Studies.”

You likely saw many news reports about this study concluding that giving birth increases breast cancer risk. It’s important to know that the final answer is more complicated and nuanced than the headlines make it sound.

Pregnancy and breast cancer risk

Earlier research found that a woman’s short-term risk of breast cancer increases for 2 to 15 years after a pregnancy, according to Ann Partridge, M.D., MPH, a medical oncologist at the Dana-Farber Cancer Institute and director of the Program for Young Women with Breast Cancer. But the studies were not able to figure out a definitive reason for this short-term increase in risk.

Still, if a woman’s first pregnancy happens before the age of 30, her overall lifetime risk of developing breast cancer after menopause decreases.

When breast cells are made in adolescence, they are immature and very active until a woman’s first full-term pregnancy. The immature breast cells respond to the hormone estrogen as well as hormone-disrupting chemicals in products. Your first full-term pregnancy makes the breast cells fully mature and grow in a more regular way. This is the main reason why pregnancy helps protect against breast cancer. Being pregnant also reduces your total number of lifetime menstrual cycles, which may be another reason why earlier pregnancy seems to offer a protective effect.

When a woman is older when she has her first baby or if she never has a full-term pregnancy, her breast cancer risk goes up because she is exposed to more estrogen over her lifetime. Her breast cells also take longer to fully mature.

How this study was done

For this study, the researchers used information from 15 earlier studies that included 889,944 women younger than 55. When the study started, none of the women had been diagnosed with breast cancer. The women were followed for 24 years or more.

During the study, 18,826 women (2%) were diagnosed with breast cancer.

Women who had given birth had a slightly higher risk of breast cancer compared to women who had never given birth. This increase in risk was highest about 5 years after giving birth and then declined as time went on. By 24 years after giving birth, women who had given birth had a lower risk of breast cancer than women who had never given birth.

It’s very important to know that the risk for breast cancer in both groups of women — those who gave birth and those who didn’t — was low, as was the increase in risk for women who had children.

  • Women who had given birth had a 2.2% risk of being diagnosed with breast cancer.
  • Women who had never given birth had a 1.9% risk of being diagnosed with breast cancer.

So the difference in risk between the two groups was only 0.3%.

What the results mean for you

“This research is important because it shows that breast cancer risk factors for young women can be different from risk factors for older women,” said study co-author Hazel Nichols, assistant professor of epidemiology at the University of North Carolina, in an interview. “[But it] shouldn't cause alarm. Even with the increase we saw, breast cancer risk for women in this age group is still low overall.”

It’s also important to keep several things in mind when thinking about this study:

  • While this study showed somewhat of a link between pregnancy and breast cancer, it was not designed to figure out what was causing the link. Most experts believe that giving birth does not directly increase breast cancer risk. It’s possible that weight gain after a pregnancy could be contributing to the small increase in risk.
  • The study included only women who were younger than 55. Breast cancer risk increases as a woman ages, and most breast cancers are diagnosed in women who are older than 50. So the study looked at a group of women who were already less likely to be diagnosed with breast cancer.
  • Other factors are likely more important than pregnancy history when determining your overall breast cancer risk. A family history of breast cancer or a known genetic mutation linked to breast cancer are probably stronger factors in your personal risk profile. If you don’t know your personal risk of breast cancer, it makes sense to talk to your doctor about scheduling time to talk about risk factors and use a tool or calculator to estimate your risk. Once you know your risk, you and your doctor can develop a breast cancer screening plan that meets your individual needs and gives you peace of mind.

The decision to have a child is very personal, complicated, and requires commitment and support. The inability to have children can be very painful, and the prospect of having a child without a partner may be daunting for some women.

Some women may choose not to have children. Others may choose to wait until they are older to have children. Many women who would like to get pregnant are stopped by infertility. After a breast cancer diagnosis, the opportunity to have children can be hindered by lingering treatment side effects (including infertility) and taking hormonal therapy medicine to reduce the risk of the cancer coming back (it's unsafe to take hormonal therapy while you're pregnant).

If having children earlier rather than later is an option for you, you may want to do that. Still, this is a highly individual decision affected by many factors besides breast cancer risk.

Whether or not you have children at a younger age, there are lifestyle choices you can make to keep your breast cancer risk as low as it can be, including:

  • maintaining a healthy weight
  • exercising every day
  • limiting or avoiding alcohol
  • eating a diet that’s low in processed foods, sugar, and trans fats
  • not smoking

To learn more about breast cancer risk and other options to keep your risk as low as it can be, visit the Breastcancer.org Lower Your Risk section.

Written by: Jamie DePolo, senior editor

Reviewed by: Brian Wojciechowski, M.D., medical adviser


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