Pregnancy And The Future Risk Of Breast Cancer

The hormonal changes in a woman's body that occur during pregnancy affect her risk of cancer for the rest of her life, not just while she's pregnant. 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 the hormone 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 while breastfeeding, however, women do not menstruate — reducing their lifetime exposure to estrogen, and thereby incidentally reducing their lifetime risk of breast cancer. The effects of pregnancy on a 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 when compared to 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.

Abortion, Miscarriage, And Other Factors Influencing Fewer Risk Of Breast Cancer

Pregnancy, of course, is not the only experience of a woman's life that changes her estrogen and progesterone levels. Hundreds of millions of women of reproductive age take the contraceptive Pill, which is usually a combination of estrogen and progesterone, as the pill alters a woman's hormonal balance to prevent pregnancy. Taking the pill raises a woman's risk of breast cancer after menopause by about 25%.

Many women further take estrogen replacement therapy to deal with intimate dryness, hot flashes, and mood swings after the menopause. In women who have not yet reached the age of 65, estrogen replacement therapy raises a woman's risk of breast cancer about 35%. After the age of 65, however, most doctors believe the risk of cancer is lower. In the Women's Health Study, taking a combination of estrogen and progesterone replacement therapy, instead of taking estrogen alone, was found to cause even greater risk of breast cancer.

Beginning menstruation relatively early in life and experiencing menopause relatively late in life increase a woman's cumulative exposure to estrogen and her lifetime risk of breast cancer, because these women will experience more menstrual cycles during their lifetimes (unless other factors prevent them from menstruating, including repeated pregnancy, prolonged breastfeeding, and the use of hormonal contraceptives). Women who have their first periods before the age of 13 or who continue menstruation after the age of 50 have an approximately 100% greater risk of breast cancer. (It is important to understand that though that number is huge, it does not mean every woman who meets these conditions will definitely develop breast cancer!)

Women who have had uterine or ovarian cancer are at greater risk of developing breast cancer, and women who have previously had any form of breast cancer, even if "in situ," non-metastatic breast cancer are at greater risk of developing metastatic, potentially fatal breast cancer later in life. There is some disagreement among experts as to whether women who have large breasts (breast hyperplasia) or who have fibrocystic breast disease are at greater risk of cancer.

But what about abortion and miscarriage?

Does failing to carry a baby to term somehow increase the risk of breast cancer later in life?

Although opponents of abortion maintain that women are somehow "punished" for failing to carry a baby to term, the medical evidence suggests that women who undergo miscarriages or induced abortions are only as likely to develop cancer as women who have not become pregnant at all. There were two studies in the 1990's that suggested a link between abortion and cancer, but later examination of the research found that the investigators sought out replies from women whose moral views supported their own. Whatever one believes about the morality of abortion, the scientific fact is that it does not "cause" breast cancer — although an abortion certainly does not prevent it, either.

Women who have children also enjoy lower rates of ovarian cancer — but it is important to put the risk of cancer in perspective. In the United States of America, about 1 woman in 8 is eventually diagnosed with breast cancer. Even doubling her risk of breast cancer only increases that probability to 1 in 4. On the other hand, even women who have had babies, who have never used hormonal birth control, and who never had abortions can develop the disease. Cancer can strike almost anyone, although it is possible to lower the risk.

No woman will have as many children as possible simply to reduce her risk of developing breast cancer — high-risk women are far more likely to opt for a double mastectomy. However, women who experience more than a few pregnancies do a lot of hard work and place their body through a lot of risks, so the thought of being "rewarded" with a lower cancer risk may be comforting.