The Complexities of Hormones, Motherhood, and Breast Cancer

How do pregnancy and breastfeeding affect your breast cancer risk?

mother and baby girl
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While motherhood is often a complicated mix of joy and stress, some women face the added shock of a breast cancer diagnosis. Women of any age or stage of life can be diagnosed with cancer—it is not limited to the central theme of pregnancy and breastfeeding.

Hormones—particularly the female hormones estrogen and progesterone—and breast cancer have a very complex relationship. Fluctuations in female hormones happen every month throughout a woman’s reproductive years and diminish after menopause. A woman’s lifetime exposure to hormones, typically measured in the number of menstrual cycles, is a modest risk factor for breast cancer.

However, there are many factors related to pregnancy and lactation that can reduce the risk of breast cancer. But for 1 in 3,000 women, breast cancer during or soon after pregnancy becomes a reality.

In a discussion with the Breast Cancer Research Foundation (BCRF), Elisa Port, MD, author of “The New Generation Breast Cancer Book,” shared valuable insights on the far-reaching implications of a breast cancer diagnosis during or after pregnancy. She also commented on how women can be vigilant about health and the newest treatment options available. Dr. Port is a BCRF investigator and Chief of Breast Surgery and Director of the Dubin Breast Center at Mount Sinai Hospital in New York.

Development of the Breasts During Pregnancy and Breastfeeding

The full process of pregnancy to breastfeeding and finally, involution, or when the breast returns to its pre-pregnancy state, causes the breast cells to mature and changes the kinds of hormones that are active in the breast.

The process reduces exposure to estrogen. It is believed that the decrease in estrogen exposure and mature breast cells reduces the likelihood that cancer can develop. This is why the early-age of first birth and number of births (known as parity) can slightly reduce the risk of breast cancer later in life when a woman is in their 50s, 60s or 70s.

Breastfeeding Reduces Overall Breast Cancer Risk

Breastfeeding also causes changes in hormone levels. Because it delays the return menstruation after pregnancy, it reduces a woman’s usual exposure to estrogen during her monthly cycle, lowering the risk of breast cancer, especially in premenopausal women.

Compared to other countries, breastfeeding in the U.S. remains low, particularly for black women as compared to white women: 62 percent of black women compared to 79 percent white women started breastfeeding soon after birth in the latest data available from 2014. However, at 6 months after birth, only 36 percent of black moms compared to 52 percent of white moms were still breastfeeding. This is particularly concerning, as a recent report found that black women were more likely to die from breast cancer than their white counterparts, a disparity that is increasing across the country.

A pooled analysis of 47 studies, comparing mothers who have breastfed to mothers who never breastfed, found that:

  • Mothers who breastfed for a lifetime total (combined duration of breastfeeding for all children) of one year were slightly less likely to get breast cancer.
  • Lifetime total of two years got about two times the benefit/risk reduction than those who breastfed for one year.

A study released in the Annals of Oncology found that breastfeeding may be especially good at lowering the risk of ER-/ triple negative breast cancer, a particularly aggressive form of the disease.

Not only does breastfeeding reduce the risk of breast cancer later in life and have numerous nutritional benefits for the baby, it also helps mom by lowering the risk of ovarian cancer, postpartum depression, and even type 2 diabetes.

Pregnancy-Related Breast Cancer

In rare cases, pregnancy might trigger breast cancer. Breast cancer that occurs during or soon after a pregnancy is often found at a more aggressive stage, in part because most women are not looking out for it and are diagnosed later, or can be misdiagnosed for other issues related to breastfeeding, such as mastitis.

Here are a few ways you can be vigilant about your breast health:

  • Always know your “normal.” Be familiar with your body and any changes that happen before, during and after pregnancy (and generally throughout life, starting as a teenager).
  • If there is anything suspicious, be sure to bring it to your doctor’s attention. This includes signs other than lumps, such as nipple inversion, dimpling of the skin, or discharge from the nipple other than breast milk.
  • Risk of breast cancer increases for about four to five years after each pregnancy. A general rule of thumb is that women should be particularly vigilant about unusual changes in their breast until their child enters kindergarten.

Treatment for Pregnancy-Related Breast Cancer

When breast cancer occurs during pregnancy, treatment becomes more complicated. Whether to have surgery and/or chemotherapy may depend on the trimester.

Second and third trimester diagnoses typically have the best outcomes for having a viable pregnancy and treating the breast cancer. Treatment during the first trimester is the trickiest and most complicated time to decide on treatment options, so it’s important to balance treatments to the mother with the potential risks to the fetus.

Treatments for Hormone-Driven Breast Cancer

Because of BCRF-funded research by Dr. Charles Perou and many other scientists around the globe, we now know that breast cancer is not one disease, but many. Doctors can determine which type of breast cancer a woman has by analyzing the proteins and DNA in the tumor cells removed by biopsy and by full-pathology after surgery.

Treatment generally follows a regimen of surgery, radiation and/or chemotherapy, but knowing the subtype has fueled the development of targeted therapies that can better personalize treatments for each patient’s tumor. 

Targeted therapies for each subtype include:

  • Triple-negative breast cancer (TNBC), which makes up 10 to 15 percent of breast cancers: There are no targeted therapies yet approved for TNBC. However, chemotherapy cocktails can be effective in treating TNBC and in some cases create robust responses.
  • HER2+ breast cancer, which makes up 15 to 20 percent of breast cancers: HER2+ was an aggressive diagnosis until a drug called Herceptin was developed.
    • After Herceptin was approved in 1998, HER2+ breast cancer went from being life-threatening to treatable, maybe even curable for some patients.
    • HER2+ breast cancer can recur in a new organ, and may become resistant to Herceptin. New drugs to treat HER2+ include Tykerb, Perjeta, and Kadcyla (T-DM1), which may help overcome Herceptin resistance.
    • In a recent study involving metastatic breast cancer, patients with HER2+ breast cancer had better overall survival (56 months versus 40 months) when Herceptin was combined with Perjeta, compared to patients receiving Herceptin alone.
  • ER+ breast cancer, 60 to 70 percent of breast cancers: ER+ is the most common form of breast cancer and impacts about 60 to 70 percent of the new 250,000 cases each year. It can be treated with surgery, radiation, chemo PLUS anti-estrogen therapies. We’ve made great advances in treating ER+ breast cancer; new drugs known as mTOR inhibitors and CDK4/6 inhibitors target specific proteins and are showing promise in clinical trials.

    Fertility After Treatment

    While fertility after a breast cancer diagnosis is a major concern, many women have successful pregnancies after breast cancer. Most treatment plans include a regimen of drugs like tamoxifen for five to 10 years after surgery and/or chemotherapy to reduce the risk of recurrence and since the drug can cause birth defects pregnancy should be avoided. Some anti-estrogen therapies are given with a drug that suppresses ovarian function to preserve fertility. Freezing eggs or embryos is also common and may be a recommended option.

    Options must be discussed before treatment so that a woman can make the best choice for her and her family.

    Key Takeaways

    • Pregnancy and breastfeeding generally reduce the risk of breast cancer later in life.
    • The risk for breast cancer increases slightly for five years after each pregnancy. Be vigilant about your breast health during this period.
    • If breast cancer occurs during or after pregnancy, treatment is a complex balance of potential risk to the mother and fetus.
    • New, targeted therapies for hormone-driven breast cancer has improved the effectiveness of treatment. 
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