Pregnancy is energetically expensive and may push mothers to the limits of human energy expenditure and/or energy absorption (maternal metabolic limits hypotheses), particularly when coupled with high levels of physical activity. This project assesses how mothers manage the energy costs of pregnancy and how maternal and fetal health outcomes are the result of energy allocation within a limited budget. Exercise is generally thought to be beneficial during pregnancy, but evolved energetic limits may substantially impact reproductive outcomes in highly active mothers. High levels of physical activity during pregnancy may cause mothers to approach their metabolic limits earlier in pregnancy, thereby reducing energy available for investment in the baby or for other physiological tasks, such as maternal fat storage. Birth could be triggered when a mother can no longer meet the energy demands of both fetal growth and maternal metabolism. I will measure energy metabolism and physical activity over pregnancy in female endurance runners (N=30) to provide the first direct test of the maternal metabolic limits hypotheses. By exploring pregnancy physiology, this project will advance our understanding of how uniquely human birth characteristics have evolved, beyond traditional maternal size constraint hypotheses like the obstetrical dilemma. Results will address strategies to promote physical activity and reproductive health for optimal pregnancy outcomes.