Pregnancy consists of a series of small, continuous physiologic adjustments that affect the metabolism of all nutrients. The adjustments undoubtedly vary widely from woman to woman depending on her prepregnancy nutrition, genetic determinants of fetal size, and maternal lifestyle behavior. Studies of protein and energy metabolism illustrate the potential of adjusting the use of those nutrients to conserve a fetal supply. Adjustments in the metabolism of nitrogenous compounds are in place by the second quarter of pregnancy. During the last quarter of pregnancy, when fetal demands are greatest, those adjustments allow a positive nitrogen retention. The energy requirement of basal metabolism is influenced by maternal prepregnant nutrition and by fetal size. If maternal energy reserves are low at conception, the basal metabolic rate is down-regulated to conserve energy. Also, women having larger babies tend to have greater increases in their basal metabolic rate and lower rates of maternal energy storage. Changes in maternal food and physical activity behaviors during gestation may augment the physiologic adjustments. However, the substantial variability in food intakes and physical activity makes it difficult to show those changes. Thresholds in the capacity to adjust nutrient use to the amount supplied exist for all nutrients. When intakes fall below the threshold, fetal growth and development is affected more than is maternal health. Efforts to achieve good maternal nutritional status preconception as well as throughout gestation best assure a good milieu for fetal growth and development.