The aim of this review was to elaborate a conceptual framework of the most important aspects of the main biochemical processes of synthesis and breakdown of energy substrates that human and pig foetuses and newborns can use during the transition from foetus to newborn. Under normal physiological conditions, the growth and development of the foetus depends upon nutrients such as glucose, lipids and amino acids. In addition to the maternal and foetal status, genetic factors are also reported to play a role. The main function of the placenta in all species is to promote the selective transport of nutrients and waste products between mother and foetus. This transport is facilitated by the close proximity of the maternal and foetal vascular systems in the placenta. The foetus depends on the placental supply of nutrients, which regulates energy reserves by means of glycogen storage. Also, the synthesis of foetal hepatic glycogen guarantees energy reserves during perinatal asphyxia or maternal hypoglycaemia. However, the foetus can also obtain energy from other resources, such as gluconeogenesis from the intermediary metabolism of the Krebs cycle and most amino acids. Later, when the placental glucose contribution ends during the transition to the postnatal period, the maturation of biological systems and essential metabolic adaptations for survival and growth is required. The maintenance of normoglycaemia depends on the conditions that determine nutrient status throughout life: the adequacy of glycogen stores, the maturation of the glycogenolytic and gluconeogenic pathway, and an integrated endocrine response.