The effect of antenatal steroid administration on the fetal response to vibroacoustic stimulation.

Background: Betamethasone transiently suppresses multiple fetal biophysical activities, including breathing movements, limb and trunk movements, heart rate variability, and heart rate accelerations. Unnecessary iatrogenic delivery of preterm fetuses due to the false diagnosis of fetal compromise has been described in this setting. The sonographically observed startle response of the fetus to vibroacoustic stimulation has been described as another modality to provide reassurance about fetal well-being. It is unknown, however, whether the startle response is also suppressed by betamethasone. The purpose of this study was to examine the effect of betamethasone on this biophysical parameter.

Methods: A prospective cohort study. Vibroacoustic stimulation was applied to the maternal abdomen and fetal movement responses were sonographically observed prior to (0 hours), 48 hours after, and 96 hours after betamethasone administration. We recorded the presence or absence of the fetal startle response, and, if a response was present, graded semi-quantitatively the intensity of the movements (vigorous versus sluggish).

Results: Twenty-two of 26 fetuses (84.6%) displayed a vigorous vibroacoustic startle response prior to betamethasone administration, in comparison to three of 26 fetuses (11.5%) at 48 hours after exposure (p<0.0001). Eleven fetuses and eight fetuses displayed no startle response at all (p<0.0005), or a sluggish response only (p<0.0005) at 48 hours, respectively. At 96 hours after betamethasone exposure, no differences in the number of fetuses with a vigorous, sluggish, or absent response were observed in comparison to 0 hours. Stratification of cases by gestational age groups of 28-30 weeks versus 31-34 weeks showed similar response patterns.

Conclusions: Antenatal betamethasone exposure transiently suppresses the sonographically observed fetal startle response to vibroacoustic stimulation. Accordingly, this modality cannot be used for the ascertainment of fetal well-being of steroid exposed fetuses. Betamethasone seems to suppress central nervous system dependent biophysical activities. including the brain-stem dependent vibroacoustic startle reflex.