Estimated cardiac vagal tone predicts fetal responses to mother's and stranger's voices

Heart rate responses of 84 near‐term fetuses to recorded female voices were examined in 166 trials of auditory stimulation. Each fetus was presented with a 2‐min recording of their mother's voice and a 2‐min recording of a female stranger's voice, in counterbalanced order, with a 10‐min rest period between trials. High frequency heart rate variability during a 2‐min baseline period was used to estimate cardiac vagal tone for each trial. Differential heart rate responses to familiar and unfamiliar voice recordings were observed during a 2‐min poststimulus period, only when estimated cardiac vagal tone was high. This finding suggests that vagal tone plays a moderating role in the cardiac responses of term fetuses to familiar and unfamiliar stimuli. © 2007 Wiley Periodicals, Inc. Dev Psychobiol 49: 543‐547, 2007.