Ultrasound assessment of fetal hearing response to vibroacoustic stimulation.

Assessment of fetal responses to external stimuli could be a vital clue for understanding development of fetal neurophysiology, which is extremely challenging to explore. To study hearing development in growing human fetus, we assessed sonographic fetal movement responses to external auditory stimulus at increasing period of gestation. In 123 normal pregnant women between 16 and 40 weeks' gestation, sonographic assessment of fetal movements (gross body movement, isolated limb movement, breathing movement and startle response) was carried out before and after administering vibroacoustic stimulation (VAS). Types and number of fetal movements during 5-min period each - immediately before and after application of VAS - were compared. With increasing gestational age, spontaneous gross body movement decreased significantly between 16-28 and 29-40 weeks of gestation (93.3 versus 66.6%;  < .001). However, VAS significantly increased gross body movement at 29-40 weeks of gestation (66.6 versus 93.6%;  < .001). Incidence of isolated limb movement was inversely related to increasing gestational age. However, VAS was associated with significantly increase in isolated limb movement during 29-40 weeks' gestation (57.1 versus 80.9%;  = .007). VAS induced almost similar pattern of response for both fetal startle and breathing movements. Fetal movement responses to VAS are consistent after 28 weeks' gestation. These findings suggest fetal hearing develops at or before 28 weeks of intrauterine life.