Age at First Pregnancy and the Risk of Breast Cancer

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Your age at the time of a first pregnancy can lower the risk of breast cancer. You've probably heard this statistic, but what ages are we talking about, and why would this be true?

Most of the benefits of having a baby are intangible, emotional, and social. But here's some evidence that pregnancy gives you a gift of health—an edge against developing breast cancer.

Pregnancy and Breastfeeding Lower Estrogen Exposure

Pregnancy before age 30 and breastfeeding reduces the total number of lifetime menstrual cycles, which is thought to be one reason they help lower your risk. The hormone estrogen fuels 80% of all breast cancers. Since pregnancy and lactation reduce your estrogen levels, your risk is decreased each time you are pregnant and while you are nursing your baby, at least up to a point.

Does Age At Pregnancy Matter?

According to the National Cancer Institute, having a full-term pregnancy at or before age 20 offers the most protection against developing breast cancer. This may cut your risk of breast cancer in half relative to women who have a child at the age of 35 or older or never have children. Breastfeeding keeps your estrogen levels low, so you don't have pre-pregnancy levels of estrogen until your baby is weaned.

Having your first pregnancy at age 30 or older offers less protection against breast cancer. Alpha-fetoprotein, a protein produced by the fetus during pregnancy, helps regulate fetal growth. It can also help suppress breast cancer cells. Over the age of 30, alpha-fetoprotein works differently, and may actually help promote rather than inhibit breast cancer development.

How Pregnancy Helps Prevent Breast Cancer

Breasts are developing during puberty, when hormone levels are changing rapidly and body-wide maturation is taking place. Breast tissue cells reach complete maturity after a full-term pregnancy. Your breasts are immature from your first menstrual cycle to your first pregnancy. Researcher Irma Russo, MD of Fox Chase Cancer Center in Philadelphia believes that limiting the time that breast cells are immature offers the best protection against cancerous changes. A hormone produced during pregnancy, human chorionic gonadotropin (hCG), causes breast cells to mature and protects against future cancer development. The pregnancy hormone hCG actually causes permanent genetic changes in your mammary glands, and these genetic changes can help prevent breast cancer.

During pregnancy, fetal cells are produced, and those cells may stay in your peripheral circulation for a long time after your pregnancy. The ability of these persistent cells floating about in your bloodstream is called fetal microchimerism (FMc). Research done by Dr. Vijayakrishna K. Gadi of the Fred Hutchinson Cancer Research Center in Seattle shows that these fetal cells may help reduce your risk of breast cancer. Fetal microchimerism may cause a protective effect by causing your immune system to be alert for malignant (cancerous) cells to destroy. "We have known for some time that pregnancy can be protective for breast cancer," Gadi said, "but our results help to address the enigma of why not all women are protected." But the researchers are hopeful for future practical application for their results. "With further studies," added Gadi, "we might be able to develop these fetal cells as a form of treatment for breast or even other types of cancers."

Pregnancy Is Not a Guarantee of Protection

Pregnancy cannot, however, offer full protection against breast cancer. It is possible to be diagnosed with breast cancer during pregnancy and to be treated with chemotherapy. A diagnosis of breast cancer during pregnancy is rare: only one in 3,000 (0.03%) to one in 10,000 (0.01%) pregnant women are found to have breast cancer. There is a very noticeable increase in breast cancers for the first year after pregnancy, which afterward then drops significantly below rates for those who have never been pregnant and never given birth.

Pregnancy and Fertility After Breast Cancer

If you are diagnosed with breast cancer, chemotherapy and follow-up medications such as estrogen suppressors and aromatase inhibitors can cause your ovaries to stop working for a while. During this time, you may be temporarily infertile, but if you are not yet menopausal after treatment, your fertility may return 6 to 12 months after chemotherapy is complete. You also have the option of freezing eggs or embryos before you begin treatment, which is important as there is no guarantee regarding future fertility. The American Cancer Society website states, "Despite concerns that pregnancy could cause cancer to return, studies to date have not shown this to be true for any type of cancer." Most breast cancer survivors who wish to have children after treatment worry about the pregnancy's hormonal changes causing a recurrence, but studies have shown no difference in recurrence with or without post-treatment pregnancies.

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