MOTOR RESPONSIVITY DURING HABITUATION TESTING OF NORMAL HUMAN FETUSES

Features of fetal motor responsivity include both the magnitude of the startle response elicited by a single stimulus (i.e., basal reactivit y) and the ease by which responding to successive stimuli is inhibited (i. e., habituation). We examined basal motor reactivity and habituat ion of the motor response in 56 normal human fetuses between 34 and 40 weeks of gestation. Testing consisted of 8 trials of a 1-sec vibroaco ustic stimulus (VAS) with a 10-sec interstimulus interval. A score of 0-10 was assigned for each trial based on subjective assessment of int ensity and duration of the fetal motor response. Measures of habituati on included the ratio of responding after a fixed number of trials div ided by the initial response, and the rate of change in the behavioral response over trials. No relationship was found between the rate of m otor habituation and either basal reactivity, gestational age, or pres timulus fetal heart rate (FHR) variability. In contrast, more mature f etuses responded less intensely to the first stimulus than did their y ounger counterparts (r = -0.329, p = 0.005), and fetuses who were init ially in a quiet state exhibited a more vigorous startle response as c ompared to fetuses who were initially in a more active state (r = -0.3 72, p = 0.001).