Published data from chemical analyses of human fetuses have been utilized to construct a reference fetus of representative body composition. For gestational ages 24 through 40 weeks, body composition of the reference fetus is presented, including water, lipid, protein and major minerals. Concentrations of water, sodium, and chloride per unit of body weight decrease with increasing gestational age, whereas those of protein, lipid, calcium, phosphorus, magnesium and potassium increase. From the estimates of body composition at each age and from the gain in body weight, composition of gain and daily increments of body components have been calculated.