Fetal habituation to vibroacoustic stimulation in relation to fetal states and fetal heart rate parameters.

OBJECTIVES: Fetal habituation to repeated stimulation has the potential to become a tool in the assessment of fetal condition and of the function of the fetal central nervous system (CNS). However, the influence of fetal quiescence and activity on habituation remains to be clarified. We studied habituation and the influence of fetal state and fetal heart rate (FHR) parameters on habituation in healthy term fetuses.

SUBJECTS AND METHOD: We studied habituation in 37 healthy fetuses in two tests with an interval of 10 min. The vibroacoustic stimuli were applied to the maternal abdomen above the fetal legs for a period of 1 s every 30 s. A fetal trunk movement within 1 s after stimulation was defined as a positive response. Habituation rate is defined as the number of stimuli applied before an observed non-response to four consecutive stimuli. The FHR patterns (FHRP) of the 10 min observation period before and after the tests were visually classified. Fetal states were defined according to the FHRP. Baseline FHR, FHR variability and the number of accelerations were calculated in a subgroup of 25 fetuses.

RESULTS: Of the 32 fetuses that responded normally during the first test, 26 habituated and six had persistent responses. The median habituation rate decreased significantly in the second test (P=0.001). There was no difference in habituation rate between fetuses that where initially in a quiet state and those in an active state. The FHR parameters before the first test and the difference between these FHR parameters before and after the test did not correlate with the habituation rate.

CONCLUSIONS: Although the majority of healthy fetuses was able to habituate, the interfetal variability in habituation performance is such that testing of habituation seems not to be a sensitive tool for the assessment of the fetal CNS. This variability is neither the result of differences in fetal state nor of the various FHR parameters before testing, nor of the difference in change of FHR parameters arising from stimulation.