Recent studies report that pregnancy produces a protective effect against breast and ovarian cancers. The more pregnancies you go through — and the younger you start having babies — the greater the effect; some research has found that breastfeeding for more than three months can also lower the risk of certain cancers.
Right now, the pregnancy/breast-cancer-reduction relationship is based on theories, says Kevin Hughes, MD, director of the Breast Center at the Lahey Clinic in Burlington, Massachusetts. One hypothesis, based on the fact that ovulation ceases during pregnancy, suggests that women who ovulate less over a lifetime are less likely to develop breast or ovarian cancer. Another theory suggests that breast tissue that never goes through pregnancy and breastfeeding may also be more prone to breast cancer, even though the tissue is hormonally stimulated to prepare for future milk production during each menstrual cycle.