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The hormonal changes in a woman's body that occur during pregnancy affect her risk of cancer for the rest of her life. Some hormonal changes during pregnancy reduce the risk of the future development of cancer. Some of the hormonal changes during pregnancy increase the risk of future development of cancer. On the whole, however, women who get pregnant earlier in life, and who get pregnant more times during their lives, enjoy lower rates of cancer both before and after menopause.
Why Would Pregnancy and Breastfeeding Reduce a Woman's Risk of Cancer?
Many kinds of cancer and most cases of women's breast cancer are "fed" by estrogen. A woman's estrogen levels soar during the first half of her menstrual period, causing the thickening of the lining of the uterus to prepare it for the possibility of conceiving a baby. During pregnancy and later during breastfeeding, however, women do not menstruate, reducing their lifetime exposure to estrogen, and reducing their lifetime risk of breast cancer. The effects of pregnancy on woman's risk of breast cancer are not uniform throughout her life, however.
Pregnancy-Related Reductions in Breast Cancer Risk
There is clear evidence that women who have their first full-term pregnancy before the age of 20 have about half the risk of one kind of breast cancer later life of women who have their first full-term pregnancy after the age of 30. However, the reduction in risk only applies to estrogen receptor-positive breast cancer, a kind of cancer that is activated by estrogen. Having a baby in her teens does not protect a woman from estrogen receptor-negative breast cancer, which is not influenced by a woman's estrogen levels.
There is also clear evidence that women who have more children are less likely to develop breast cancer later in life than women who have fewer children. Women who bear five or more children have half the breast cancer risk of women who never have children. (A well-publicized recent study suggests that the protective factor may be much stronger, that women who have five or more children may have only 15% of the risk of breast cancer facing women who have had no children at all.)
Women who survive a potentially life threatening condition called pre-eclampsia during their pregnancies have about a 30% decreased risk of developing breast cancer.
Breastfeeding also protects against breast cancer. Nursing a child for a year or more protects against both estrogen receptor-positive and estrogen receptor-negative breast cancer. Scientists believe that nursing causes more cells in the breast to differentiate, that is, to mature in ways that make their DNA more stable and less susceptible to cancerous changes.
Pregnancy-Related Increases in Breast Cancer Risk
The protective effects of pregnancy and breastfeeding on breast cancer risk are long-term. In the short term, the massive amounts of estrogen released during pregnancy actually make women more likely to develop pre-menopausal breast cancer, although it is very rare for women to develop breast cancer before the age of 25 and unusual for women to develop the risk of breast cancer before 50.