Nulliparity is associated with increased reproductive malignancies and early first full-term pregnancy has been found to reduce risk of breast cancer. There is also evidence that increased weight in childhood is protective against breast cancer. This research focuses on body size at different time points across the lifecourse and its effect on breast cancer risk, to understand the time frame in which undifferentiated nulligravid breast is most susceptible to carcinogenic insults. However, separating the effects of risk factors at different stages of the lifecourse is challenging due to confounding in conventional epidemiological settings. This is a key motivation behind using a Mendelian randomization (MR) approach. Conventionally, MR studies use a single measurement to estimate the effects of an exposure on an outcome. Effects obtained are therefore often interpreted as the lifetime effect of the genetically predicted exposure. This research will exploits the notion that genetic associations may arise from the direct effects of the same inherited variants at different stages throughout life. Our aim is to assess the effect of body size between menarche and first birth at different intervals across the lifecourse on breast cancer risk in later life.