Maternal obesity in pregnancy is increasing worldwide and is associated with adverse pregnancy outcomes for both mother and baby. However, the risks are heterogeneous, with some obese pregnancies leading to preterm birth and reduced fetal growth, and others complicated by high birth weight. Some women who are obese may alternatively have uncomplicated, healthy pregnancies. There is an urgent need to identify those women and babies most at risk of specific outcomes and thus better target healthcare management and interventions. To do this, we first need to better understand the mechanisms underlying how maternal risk factors combine with the fetal response to influence risk. To date, our work using ALSPAC (project B2388) and other studies has identified genetic variation in both mother and baby that is associated with birth weight. We have used these genetic variants to investigate causal associations between maternal modifiable risk factors (e.g. blood pressure, glucose levels) and birth weight of the baby. However, many questions remain unanswered, including whether maternal blood pressure or glucose also influence the timing of birth, the weight of the placenta and the levels of insulin (a key growth factor) produced by the fetus. In addition, the role of the fetal response to the maternal environment is not well defined, and it is not known whether this fetal response influences maternal metabolism. This project will transform our understanding of the mechanisms connecting maternal BMI, glucose and blood pressure, fetal and placental growth, fetal insulin and the timing of birth, using large-scale genetic datasets. By clarifying these mechanistic relationships, the work will pave the way for the identification and targeted management of high-risk obese pregnancies.