Fetal facial reactions and response decrement patterns to external noise stimulation were studied to characterize normal fetal neurobehavior in the third trimester. Response decrement, or habituation, is thought to reflect higher central nervous system function. Two hundred women with uncomplicated pregnancies, who were subsequently delivered of healthy infants at term, were studied between 26 and 41 weeks' gestation. After ultrasound views of the fetal face were obtained, a vibratory acoustic stimulus was applied repetitively to the maternal abdomen near the fetal head. Response decrement was defined as cessation of all components of the facial reaction, except eye blinking, over two sequential stimuli. Three response patterns were scored: (1) no startle, (2) startle without response decrement, and (3) response decrement by 12 stimuli. Whereas only 53% of fetuses between 26 and 27 weeks displayed startle reaction, all fetuses displayed startle responses by 28 weeks. As gestational age advances, an increasing number of fetuses exhibit response decrement, from no decrement at 26 to 27 weeks to 100% decrement at 40 to 41 weeks. A maturation of neurobehavioral response patterns takes place in normal third-trimester pregnancies. Response decrement testing may be a useful tool for in utero neurologic evaluation.