Background: Term fetuses discriminate their mother's voice from a female stranger's, suggesting recognition/learning of some property of her voice. Identification of the onset and maturation of the response would increase our understanding of the influence of environmental sounds on the development of sensory abilities and identify the period when speech and language might influence auditory processing. Aim: To characterize the onset and maturation of fetal heart rate response to the mother's voice. Methods: 143 fetuses from 29 to 40 weeks gestational age (GA) received a standardized protocol: no-sound pre-voice baseline (2 min), audio recording of their mother reading a story (2 min), no-sound post-voice (2 min). The voice was delivered 10 cm above the maternal abdomen at an average of 95 dB A; heart rate was recorded continuously. Results: For data analyses, fetuses were categorized into four age groups: 29-31, 32-34, 35-37, and greater than 37 weeks GA. Onset of response to the mother's voice occurred at 32-34 weeks GA. From 32 to 37 weeks GA, there was an initial heart rate decrease followed by an increase. At term, there was a response shift to an initial heart rate increase. The percentage of fetuses responding increased over gestation from 46% at 32-34 weeks GA to 83% at term. Conclusion: A relatively long latency and sustained duration of the heart rate response suggest auditory processing, the formation of neural networks, above the level of the brainstem.