To produce more appropriate information for evaluating fetal growth and viability, vital records data were used to compute percentile curves and perinatal, neonatal, and fetal mortality rates at specific birth weights and gestational ages. Percentile values were in good agreement with previous studies, and the large number of births (2,288,806) allowed for a more precise determination of fetal viability at various weight-age combinations than has been previously available. Mortality rates were found to be much more sensitive to birth weight than to gestational age, especially for small-for-gestational age fetuses. Optimal weight-age combinations were found to be up to 500 g and 2 weeks greater than the average combination. The results consistently emphasize the importance of rapid and sustained fetal growth at all gestational ages.