During pregnancy there is an increased demand for energy and protein to enable the fetus and placenta to grow. Current recommendations suggest an allowance for pregnancy of 6-10 g protein per day, however there is little consistent evidence that this is needed. Furthermore, there does not appear to be a simple relationship between the dietary protein intake of the mother and the size of the baby. If protein needs to be conserved, pregnant women must adapt metabolically. Methods for measuring protein metabolism have varied between studies. Some isotopic approaches are relatively invasive, but noninvasive methods are also available which can be used in larger numbers of women in free-living conditions. Taken together, isotopic studies during pregnancy indicate that there is a shift in the partitioning of amino acids towards net tissue deposition, reflected in an increased rate of protein synthesis, and away from oxidation, reflected in measures of urea synthesis. An understanding of the influences on fetal growth is needed to enable us to deal with the major problems in public health of our time. There is substantial evidence that impaired growth and development in utero is associated with a higher risk of cardiovascular and metabolic disease in adult life. In order to establish what dietary recommendations should be made, we need to explore further how women cope metabolically with the demands of pregnancy, particularly when faced with unusual demands over and above pregnancy, such as infection.