Developmental trends in fetal habituation to vibroacoustic stimulation.

Habituation is a measure of the ability to inhibit responding and is a more mature form of behavior than is persistent responding. We examined the developmental trend in habituation of the fetal startle response to repeated vibroacoustic stimulation in 90 normal human fetuses between 28 and 40 weeks of gestation. Fetal movement was graded according to the nature of the behavioral response: general startle (3), fast limb movement (2), slow rolling movement (1), and no movement (0). A significant developmental difference (p < 0.0004) in the rate of habituation was found, with response decrement occurring faster in fetuses of more than 32 weeks of gestation. Furthermore, by dividing the patients into three gestational age groups, it was determined that the greatest change in the rate of habituation occurred between 28 and 32 weeks and 32 and 36 weeks. We conclude that the rate of fetal habituation may be determined by the degree of maturation of the neural circuitry governing this form of nonassociative learning.