Reproductive factors and incidence of breast cancer: an international ecological study.

Ecological studies can help in understanding the relation of reproductive history of breast cancer. We analyzed data from 9416 women, comprising the control groups of seven countries (Australia, People's Republic of China, Colombia, (former) German Democratic Republic, Israel, Philippines, and Thailand) from the WHO international, multi-center case-control study of female cancers. Positive correlations with country-specific breast cancer incidence were observed for (median) duration of reproductive life (r > or = 0.95, p < 0.005), age at menopause (r > or = 0.84, p < 0.025) and delay to first birth (r > or = 0.59, p < 0.22) (when People's Republic of China was omitted, r > or = 0.85, p < 0.07). The association of age at first birth with breast cancer incidence was weakly positive in the whole sample (age-adjusted r = 0.18, p = 0.73), but weakly negative in the age groups 15-29 and 30-39 years and weakly positive in the age groups 40-49 and 50-64 years. A strong inverse correlation was observed between age at menarche and breast cancer incidence (r < or = -0.84, p < 0.03). These international ecological correlations agree with the associations previously reported for single populations, between higher incidence of breast cancer and younger age at menarche, older age at menopause, longer duration of reproductive life, and (possibly) longer delay to first birth. In contrast, age at first birth is only weakly related to breast cancer incidence across populations, indicating that this variable represents different constructs when measured ecologically versus individually.