Fetal auditory brainstem evoked response (ABR).

Auditory brainstem evoked responses (ABRs) were obtained in utero from five fetal lambs instrumented between 108 and 124 days gestation and tested from 2 to 24 days following surgery. The fetal ABR patterns were then compared with similar ABR measurements from term newborn lambs at 1 day of age. The characteristics of the preterm fetal ABR were dependent upon the period of gestation in which the testing was conducted. The fetal ABR is absent prior to 117 days gestation. The earliest recorded fetal ABR consisted of one or more reproducible deflections and was observable by day 117-125 of gestation. By 126 days gestation the ABR was comparable in waveform pattern to those obtained in one day old term newborn lambs but with delayed peak latencies. The most definitive characteristic of the fetal ABR was the interpeak latency between waves IV and V, which was greater than the interpeak latencies between other wave deflections. Serial recordings of fetal ABRs indicate that latencies to each wave decrease with advancing gestational age. These changes may be suggestive of a maturational process within the auditory system of the developing fetus.