Maternal nutritional status, particularly her daily protein intake in later pregnancy, plays an important role in influencing the birth outcome. The presently defined protein requirements during pregnancy are based on antenatal body protein accretion rates from the western literature, due to lack of data on the Indian population. Body composition measurements were thus done on the Indian pregnant women and their babies using a whole-body potassium counter which was constructed at SJRI. The currently available techniques of stable isotope dilution, dual energy X-ray absorptiometry, bio-electrical impedance, air displacement plethysmography and skinfold techniques are considered imprecise during pregnancy and early ages as these life stages are characterised by fluid accumulation and have limitations to radiation exposure. The newly constructed whole-body potassium counter is a safe, accurate and non-invasive method to repeatedly measure body composition at all stages of life. It measures the naturally occurring radioactive isotope of potassium (40 K) in the body. Since K is an intracellular ion predominantly and has a constant relationship with body protein in the body cell mass (BCM), the BCM or the metabolically active tissues of the body can be measured by measuring the total body potassium. Careful and accurate measurements of BCM accretion during pregnancy and neonatal body protein content with accurate measurements of maternal protein intake during pregnancy helped shed light on the effect of maternal protein intakes on maternal and fetal body protein accretion. It also helped in defining protein requirements during pregnancy based on the efficiency of protein utilization.