Pregnancy and Breast Cancer
Hormonal changes that occur during pregnancy can affect a womanís risk of developing breast cancer. For women under the age of twenty, pregnancy is protective against breast cancer, but as women age, pregnancy can actually put them at a higher risk for developing the disease.
Crane: There is a widely-held perception that pregnancy is protective against breast cancer.
Dr. Brinton: Pregnancy in the long run is beneficial.
Crane: Dr. Louise Brinton is the chief of the hormonal and reproductive epidemiology branch at the National Cancer Institute.
Dr. Brinton: But we actually see that in the short term, itís a risk factor.
Crane: She says for the first five years following a birth, a woman is at an elevated risk of breast cancer.
Dr. Brinton: We know that pregnancyís associated with a lot of changes in hormones. But we donít know which hormones could be contributing to the risks that are associated with reproductive patterns.
Crane: Researchers do know that women who get pregnant at a younger age have a lower risk of breast cancer.
Dr. Brinton: You see about a three-fold difference in risk between a woman who has a first birth at age 35, and one who has a first birth prior to the age of eighteen, so itís quite a substantial difference in risk, and we see really a linear increase in risk starting with very early ages at first birth, and then continuing to rise the longer a woman delays her first birth.
Crane: Dr. Brinton says thatís why women in developing countries, who give birth at younger ages, have typically had lower rates of breast cancer.
Dr. Brinton: But as those cultures are changing, and more and more women are delaying the ages at which they have their first birth and also having fewer children, weíre seeing rates of breast cancer rise.
Crane: Thatís happening in China, Japan, India and South America. Meanwhile women in developed countries are waiting even longer to have children.
Dr. Brinton: And of course now we have many women who are delaying their first births until after the age of thirty-five or oftentimes after the age of forty, so itís of concern that those women are probably placing themselves at a high risk of breast cancer.
Crane: Although the reasons for the risk arenít entirely clear to researchers, Dr. Brinton says they have some idea.
Dr. Brinton: We think that it may relate to the fact that you are experiencing cellular changes throughout your life and that if you wait until a late age to have a first birth, that the hormones that youíre exposed during a pregnancy can initiate those changed cells into a cancer.
Crane: Dr. Brinton says other hormonal risk factors for breast cancer include an early age at first menstruation and late menopause because of the increased exposure to estrogen over a womanís life-time. For more information on breast cancer risk factors, visit www.cancer.gov. This is Kristine Crane at the National institutes of Health, Bethesda, Maryland.