This chapter explores a critical aspect of fetal programming; namely the determinants of uterine blood flow and their role in fetoplacental development. Uterine artery (UA) blood flow can be measured directly in experimental animals but not in humans because of ethical concerns or limitations imposed by the effects of anaesthesia. Pregnancy increases UA diameter at high altitude in all groups, but within a given study the rise in Europeans is less than that seen in Europeans at low altitude or in Andeans at high altitude. Central to the hypothesis is that the Andean women's high UA blood flow during pregnancy helps maintain normal fetal growth. High altitude regions continue to provide an instructive natural laboratory for understanding fetoplacental development. Future studies are required to determine the factors responsible for raising UA blood flow during pregnancy and for explaining the UA blood flow differences between native and newcomer groups.