Maturation of responses elicited by a vibroacoustic stimulus in a group of high-risk fetuses.

The study was designed to characterize the changes in fetal heart rate (FHR) and movement responses in a group of high-risk fetuses to address the question of differential responding as a function of risk status. The study included 36 women admitted to hospital with threatened preterm labour (e.g., contractions, bleeding, premature rupture of membranes) with singleton pregnancies between 23 and 34:6 weeks gestational age. Each fetus received three vibration and three no-stimulus control trials, intermixed and randomly presented. Vibration trials consisted of a 2.5-second (s) application of the tip of a battery-powered, hand-held commercial vibrator to the maternal abdomen over the site of the fetal head while the FHR was recorded continuously and movements were scored from a cross section of the infant's abdomen viewed with a real-time ultrasound scanner. FHR within 20 s and movements within 5 s of trial onset were scored blind as to vibration or control trial. Repeated measures ANOVA demonstrated that fetuses responded to vibration with FHR accelerations from 29 weeks and movements from 26 weeks. Although movements increased with increasing gestational age, the mean peak FHR acceleration showed no significant increase from 29 to 32 weeks. From 29 weeks, the latency to response was 5 s and the peak of the acceleration occurred at about 10 to 12 s. At 26-28 weeks, a small but reliable cardiac deceleration occurred at about 5 s after stimulus onset. These preliminary findings suggest a maturation of fetal sensitivity to vibroacoustic stimulation which is similar for groups of low-risk and high-risk fetuses. However, the variability in outcome--from preterm newborn death to "healthy" term delivery--in the high-risk group suggests that the sample may be heterogeneous.