Fetal auditory brain stem response: external and intrauterine auditory stimulation.

Singleton fetuses from five pregnant ewes of 120-129 days gestation (term = 145 days) were tested for fetal auditory brain stem responses (ABR) generated by earphones placed on the maternal abdomen and compared with ABRs generated by an earphone secured in the fetal lamb's ear. To conduct these studies a cesarean section was performed on the pregnant ewe at 105-110 days gestation in order to implant stainless steel electrodes in the fetal scalp and a hearing aid receiver in the fetal external ear canal. The fetus was returned to the uterus, and the pregnancy was allowed to continue. Comparison of the externally elicited fetal ABR with the internally elicited fetal ABR indicates similar response patterns with delayed peak latencies observed in ABR responses from external stimulation. This new method of generating fetal ABRs by an external sound source documents the neurological response of the fetal auditory end organ and the fetal brain stem in the pregnant nonstressed animal model and may have applications in developmental studies of the fetal auditory system.