The acoustic stimulation test in the anencephalus: preliminary results.

It is not yet clear whether the acoustic stimulus influences the fetus by auditory or vibrational pathways. The anencephalic fetus is an interesting model for the study of the pathway for receipt of acoustic stimulation because of the near absence of cerebral hemispheres. After the traditional nonstress test (NST), the response to acoustic stimulation was assessed in eight fetal heart rate (FHR) recordings of six anencephalic fetuses of gestational age between 18 and 39 weeks. Although four preterm fetuses demonstrated nonreactive results in the NST, two term fetuses revealed reactive NSTs. However, none of these six anencephalic fetuses responded to acoustic stimulation. These data suggest that the cerebral cortex is the origin or transmission route of the FHR response to acoustic stimulation. We hypothesize that the normal fetus might receive externally applied sounds via auditory pathways rather than vibratory pathways, at least in term pregnancy.