We have been fairly unsuccessful in determining the mechanisms through which maternal manipulations are transmitted to the fetus. The rapidity of the onset of fetal responsiveness to maternal physiological alterations exceeds the temporal response curve of products of the HPA axis. Efforts to link specific changes in concurrently measured maternal physiological indicators of autonomic responsiveness (e.g., electro dermal activity, heart rate) to commensurate changes in fetal variables have been minimally successful. Instead, our interpretation of these and related findings has focused on fetal detection of and response to changes in the intrauterine milieu inspired by the manipulation. Fetal heart rate responses have been observed within seconds of disruptions of the maternal environment in the investigations of the development of fetal sensory capacities including maternal postural changes and auditory stimuli, and it is clear that sounds generated by maternal vasculature and the digestive tract are prominent in the uterine auditory environment. Specifically, we suspect that at least the initiation, if not the maintenance, of the fetal response to some maternal manipulations may be mediated by fetal perceptual detection of changes in the intrauterine milieu. The possibility of a sensory-oriented fetal response to maternal stressors has also been offered previously in nonhuman primate models. It is possible that after a certain point in gestation, when the fetal brain is suffi- ciently mature, any maternal manipulation elicits an acute phasic response that includes a rapid sensory-mediated component as the fetus detects a change, followed by a more tonic secondary response with more complex neuroendocrine, autonomic, or vasodilatory maternal input that may account for more chronic activation.