Birth weight and risk of early-onset breast cancer (Denmark)

Objective: To investigate if birth weight is associated with early-onset breast cancer. The mechanism behind an association with high birth weight could be the link between fetal growth and estrogens in utero. Methods: We conducted a population-based case–control study in Denmark including 881 women with breast cancer diagnosed before the age of 40 years and 3423 age-matched controls. Information concerning birth weight and other birth-related variables was obtained from midwife reports. Results: The risk of early-onset breast cancer was increased 1.25 times (95% CI 1.00–2.51) for birth weights above 4000 g and 1.59 times (95% CI 1.00–1.55) for birth weights below 2500 g in comparison with birthweights of 3000–3499 g. Conclusions: The finding that high birth weight is associated with breast cancer is compatible with the hypothesis that level of estrogen during pregnancy is related to breast cancer in early adult life. The finding that low birth weight is also associated with breast cancer may indicate that other characteristics of the fetal environment may be important for breast cancer in early adult life.