Reproductive factors and risk for breast cancer in Iceland.

A previous publication has shown that there has been a great increase in the incidence of breast cancer in Iceland. The present study was undertaken to evaluate the importance of known risk factors connected with the reroductive history of the women. In a prospective investigation of a cohort of 31,453 women we confirmed previous findings of the increased risk following higher age at first pregnancy and for nulliparous women. The age at first pregnancy and the proportion of nulliparous women have decreased with time in the Icelandic population so that these factors should have tended to decrease the risk, making the found increase still more important. The effect of parity is that the risk decreases with larger number of children and this effect is independent of the effect of age at first pregnancy. Lower age at menarche increases the breast cancer risk, but the contribution of that factor to the overall risk is negligible.