A new method of evaluating fetal growth.

When fetal brain weight (BrW) is expressed as a modified exponential function of total body weight (BW), there is close agreement (p = 0.9841) between measured and calculated values (Sy = 6.16 g). Fetal BrW in utero can be determined from the neurocranial cross-sectional area (CSA) of the fronto-parieto-occipital plane. The standard error of the estimate is Sy = 8.03 g. The estimation of BW from the abdominal circumference and BrW from the CSA is the basis of two indexes which reflect the BrW:BW relationship. By definition, the brain size index (BSI) is 100 in the normally growing fetus. In a given case, a value greater than 100 denotes the amount by which measured BrW exceeds the BrW which is appropriately for a normal fetus of the same BW. The somatic retardation index (SRI) expresses as a percentage the amount by which total fetal weight lags behind normal ponderal growth in cases of IUGR. The indexes provide quantitative data on relative brain and body weights and serve as a ratio scale of the severity of the pathology in IUGR.