Obesity is linked to many types of cancers. The distribution and activity of adipose tissue, which produces hormones and other potentially cancer-causing factors, vary significantly between sexes and over a lifetime. The primary objective of this project is to estimate the effect of obesity on cancer risk in women at different stages of the reproductive lifespan. Secondary Objectives include: a) investigate how weight gain during pregnancy affects cancer risk, b) examine how the obsity-associated risk varies with age, and c) assess the extent to which exogenous hormones influence the effect of obesity on cancer risk. Data from the Swedish National Registries have been provided by SCB and Socialstyrelsen for all women who have given birth since 1973 and comprise approximately 2 million women and over 4 million pregnancies.