Except during the last 2 gestational weeks, the brain is nearer its full-term size than the body is throughout prenatal development. This observation is consistent with the concept of a cephalocaudal axis of priority on the available nutritional supplies. The determination of ponderal maturity gradients shows the ascendancy of brain over somatic growth to be most marked in the first half of gestation. In absolute figures, both brain and somatic development are greatest toward the end of the intrautcrinc period when their respective weights are doubled in the last 9 and 11 weeks. Because the brain: somatic percentage index varies throughout intrauterinc development, it cannot be used as a means of determining asymmetric intrauterine growth retardation (ILJGR) in the fetus in the absence of accurate information on gestational age. When brain weight is expressed as a modified exponential function of somatic weight, the relationship is constant throughout fetal ontogeny and shows stronger covariability (p=0.9867) than the brain: body weight relationship. This method of expressing brain weight as a function of somatic weight is, therefore, useful as a means of appraising the quality of growth in cases at risk for asymmetric IUGR.