The clinical utility of concurrent measurements of the fetal biparietal diameter, as measured by ultrasonography and fetal pulmonary maturity, as assessed by the amniotic fluid foam stability index test, is reported. The clinical material consisted of 57 cases in which pregnancy produced a low birth weight neonate (less than 2500 g). The combination of a small fetal biparietal diameter (less than 8.5 cm) and a mature foam stability index value (0.47 or greater) was associated with small­for­gestational­age neonates in 16 of 19 cases. In addition, the occurrence of an immature foam stability index value (less than 0.47) and a fetal biparietal diameter of less than 8.5 cm was associated with appropriate­ for­gestational­age newborns in 16 of 16 cases. The discriminating ability of the foam stability index test in identifying the small­for­gestational­age infant was lost when the fetal biparietal diameter was 8.5 cm or more. Of clinical note was the fact that 14 cases in which the neonatal respiratory distress syndrome occurred were all associated with appropriate­for­gestational-age infants and foam stability index values of less than 0.47 (ie, immature). The results of this study suggest a potentially important clinical role for the concurrent measurement of fetal biparietal diameter and performance of the amniotic fluid foam stability index test in the prenatal evaluation of the high­risk, small, third­trimester fetus. (Obstet Gynecol 61:13, 1983)