It is well known that events occurring during a woman¿s reproductive life affect her risk of breast cancer. In contrast, our knowledge regarding the importance of exposures that took place during in utero life or early childhood and adolescence is very sparse. Such studies are difficult to perform because reliable information on what happened many decades ago is difficult to obtain. One of the main recommendations from a consensus meeting held in Washington, D.C. in January 1999 was that much more knowledge is needed regarding the importance of this early period of life both for understanding the biology of this tumor and for the identification of potential critical time periods during which the mammary cells are particularly prone to stimuli with damaging potential and to better understand the important risk factors for breast cancer development during early life.

The present proposal takes advantage of the unique opportunities in Denmark for studying aspects of early life events for risk of breast cancer. Throughout many years, there has been a tradition for registration of the population that gives excellent conditions for long-term follow-up. Based on a unique collection of annual school health records from 150,000 schoolgirls born in 1914 or later, we will study the importance of birth weight, childhood obesity, and growth patterns during childhood and adolescence for risk of breast cancer. Specifically, 4,200 of the girls who later developed breast cancer in adult life will be compared with a similar number of girls who have not developed breast cancer at the same age. The size of this study will enable us with much certainty to settle an ongoing debate of whether birth weight is positively associated with risk of breast cancer. The precise yearly registry information will also make it possible to disentangle the effects of childhood obesity, growth rate during childhood, age at menarche, and their associations with breast cancer risk in adult life. There is presently little data on this topic, whereas body stature in adult life has been associated with breast cancer risk.