Reproductive factors and breast cancer.

Abstract

Early age at menarche, late age at menopause, and late age at first full-term pregnancy are linked to a modest increase in the risk of developing breast cancer. Some evidence suggests that the earlier the full-term pregnancy, the earlier the period of decreased susceptibility of breast tissue changes begins. Nulliparity is related to an increased risk for breast cancer diagnosed after 40 years old. Multiple full-term pregnancies decrease the risk of breast cancers diagnosed after 40 years regardless of the age at first birth. On the other hand, they may increase the risk for breast cancers diagnosed before 40 years old. Surgical removal of the ovaries protects against breast cancer. Breast feeding apparently protects against breast cancer in China, but a protective effect has not been established in the US. Other than shorter intervals between menstrual periods, which tend to increase the risk, research has not yet made clear the etiologic roles of menstrual cycle characteristics. Other unclear etiologic roles include increased intervals between births, spontaneous and induced abortion, infertility, multiple births at last pregnancy, and hypertension during pregnancy. Researchers tend to accept a mechanism to explain the epidemiologic characteristics of menstrual activity and the increased risk of breast cancer, but no mechanisms have emerged for the other likely risk factors. Greater exposure to estrogen and progesterone simultaneously are linked to early age at menarche, late age at menopause, and shorter menstrual cycle length. So far, data show that long-term combined estrogen/progestin hormone replacement therapy and long-term use of oral contraceptives increase the risk of breast cancer. Moderately increased risks linked to longterm estrogen replacement therapy and obesity in postmenopausal women indicate that estrogen alone influences breast cancer risk. Since much of the research on breast cancer risk factors are inconclusive, more research is needed, especially research examining the probability of prolonged exposure to both estrogens and progesterone concurrently.