Amino acids form one of the main building blocks for fetal and neonatal growth. Despite improvements in neonatal care, including postnatal nutrition, growth faltering and suboptimal outcome after premature birth are still frequently encountered. Nutrition can partly be held responsible. Over the years, there has been a trend in delivering amino acids earlier from birth on and in larger quantities. Unfortunately, little is known about the specific metabolism of proteins, especially during fetal life or during disease. This review gives an overview of different methods of studying metabolism during early life and what we have come to learn so far. Different examples are given on the complex interplay between the placenta and the fetus. From both ovine and human studies, we know that amino acids are not only used for protein synthesis in the fetus, they are also oxidized to a large extent. Postnatally, we have succeeded in improving the nitrogen balance in preterm infants, but the preconditions need also to be improved before concluding that today's policy is optimal. Only by gaining more knowledge on both fetal and neonatal physiology and disease will we be able to further optimize growth and functional outcome in premature infants.