BIRTH-WEIGHT AS A RISK FACTOR FOR BREAST-CANCER

Background The mammary gland is largely undifferentiated before birth and may be particularly susceptible to intrauterine influences that co uld increase the risk of cancer through acceleration of cell prolifera tion or other pregnancy-related processes. Studies of migrant populati ons, animal data, and limited epidemiological evidence suggest that br east cancer may originate in utero. In a nested case-control study we assessed whether birthweight and other perinatal factors are associate d with risk of breast cancer. Methods This case-control study was nest ed within the cohorts of the two Nurses' Health Studies. We used self- administered questionnaires to obtain information from the mothers of 582 nurses with. invasive breast cancer and the mothers of 1569 nurses who did not have breast cancer (controls). Information on risk factor s for breast cancer during adulthood were obtained from the nurses; mu ltiple logistic regression analysis adjusted for these risk factors. F indings Birthweight was a significant predictor of breast-cancer risk. With women who weighed 4000 g or more at birth as the reference categ ory, the adjusted odds ratios for breast cancer were 0.86 (95% CI 0.59 -1.25) for birthweights of 3500-3999 g, 0.68 (0.48-0.97) for birthweig hts of 3000-3499 g, 0.66 (0.45-0.98) for birthweights of 2500-2999 g, and 0.55 (0.33-0.93) for birthweights below 2500 g (p for trend 0.004) . Prematurity was not significantly associated with risk of breast can cer. Interpretation Birthweight is significantly associated with breas t-cancer risk, which suggests that intrauterine factors or processes a ffect the risk of breast cancer in the offspring, High concentrations of pregnancy oestrogens may have an important role in breast carcinoge nesis, but other pregnancy hormones or intrauterine factors may also b e involved.