This chapter describes the maternal adaptation to pregnancy and the role of the placenta in nutrient transfer to the fetus. During pregnancy, an adaptation of maternal metabolism functions to ensure normal fetal growth throughout gestation and neonatal growth during lactation. The maternal metabolic reprogramming is believed to be directed by placental hormones. Maternal nutrition around the time of conception may have important effects on gestational length, fetal growth trajectory, and postnatal growth and health. Insulin-like growth factor 1 (IGF-1) is the primary fetal growth-stimulating factor in response to altered nutrient supply during late gestation and is under the control of fetal insulin. Insulin and leptin are maternal metabolic indicators that may be involved in fetal intrauterine growth adaptation and long-term health. Fetal blood sampling and the use of stable isotopes in human pregnancy have allowed for description of maternal and fetal nutrient concentrations.