Since essential fatty acids are required for normal brain development, we studied plasma lipids and EFA levels in 16 postpartum mothers (28 to 44 weeks) and in the umbilical vein and artery of 32 newborn infants. Groups of eight 24 to 33-, 34 to 37-, 38 to 42-, and 43 to 44-week-old infants were studied. Plasma fatty acid composition was studied in PL, CE, TG, and FFA by thin-layer and gas-liquid chromatography. Increased values for PL, CE, and TG (P<0.001) were noted in maternal plasma compared to cord plasma; linoleic acid was lower (P<0.001) in cord plasma PL, CE, and FFA. EFA derivatives dihomo-γ-linolenic, arachidonic, and docosahexaenoic acids were higher in cord plasma (P<0.001). Total polyenoic EFA increased with advanced gestation, and at term, was close to maternal levels. Δ-5,8,11-eicosatrienoic acid (elevated in EFA deficiency) was elevated in cord plasma as compared with maternal values (P<0.001); other criteria of EFA deficiency were absent. These data indicate that fetal EFAs are elongated and desaturated during the third trimester. These higher polyenoic acids may incorporate into lipids in the developing CNS. The lower linoleic acid levels in the fetus may be important to the transplacental transport of EFA.