Pregnancy and lactation involve extra energy needs. As extra energy intakes over pregnancy and lactation are limited, and energy savings by reduced physical activity are assumed to be restricted, it has been postulated that during pregnancy and lactation, energy expenditure is further reduced by improved efficiency of energy metabolism. Such improved metabolic efficiency could be reflected in reduced diet-induced thermogenesis, and increased work efficiency. This hypothesis was addressed in a longitudinal study, including pre-pregnant baseline measurements, measurements in each trimester of pregnancy and measurements during lactation at 2 months post partum. No changes were observed in diet-induced thermogenesis or work efficiency during pregnancy (Chapters 2 and 3, respectively) and lactation (Chapter 5). The power of the study was sufficient to detect changes of physiologically importance in both parameters. As no metabolic adaptations appear to take place, we reassessed the magnitude of the imbalance between energy inputs and outputs during pregnancy and lactation. Pronounced underestimation of the energy savings by reduced physical activity might be the major cause of this imbalance (Chapter 4). Urge differences in the changes in energy intake and in physical activity are observed between subjects. This high between-subject variability implies that energy intake recommendations based on group averages may have only limited value for individual women.