At what gestational age can a fetus start to detect sounds outside the womb?

Fetal Hearing Development Timeline

A fetus begins to detect sounds at approximately 19 weeks of gestation, with full hearing capability developing by 28 weeks of gestation. 1

Development of Fetal Hearing

• The first fetal responses to sound occur at 19 weeks of gestation, specifically to low-frequency tones around 500 Hz 1
• From 19-23 weeks, some fetuses can detect sounds, but responses are inconsistent 2
• By 24-25 weeks of gestation, sensory stimuli, including auditory stimuli, can reach the cortical level 3
• From 28 weeks onwards, all healthy fetuses demonstrate consistent responses to sound stimulation 2, 4
• The range of frequencies a fetus can hear expands with gestational age:
  • First to lower frequencies (100-250 Hz)
  • Then to higher frequencies (1000-3000 Hz) 1
• By 33 weeks, all fetuses respond to 1000 Hz tones 1
• By 35 weeks, all fetuses respond to 3000 Hz tones 1

Anatomical Development and Sound Transmission

• Thalamic afferent neurons reach the subplate zone between 20-22 weeks of gestation 3
• Thalamic afferent neurons reach the cortical plate between 23-24 weeks of gestation 3
• EEG changes in response to auditory stimuli are not present until the equivalent of 28-30 weeks of gestation 3
• Sound transmission to the fetus occurs primarily through:
  • Low-frequency sounds (below 500 Hz) which easily penetrate maternal tissues with less than 5 dB attenuation 5
  • Higher frequencies (above 500 Hz) which are attenuated by 20-30 dB 5, 6
• Sound reaches the fetal inner ear primarily through bone conduction rather than through the external and middle ear systems 5, 6

Fetal Sound Environment

• The intrauterine acoustic environment is dominated by maternal sounds:
  • Heartbeat
  • Breathing
  • Mother's voice
  • Digestive sounds
  • Body movements 2
• Background noise in utero is never below 28 dB and can rise to 84 dB when the mother is singing 2
• External conversations are audible to the fetus, though attenuated 4
• While only about 30% of phonetic information reaches the fetus, intonation patterns are almost perfectly transmitted 4

Clinical Implications

• There is evidence that fetuses can learn and remember their mother's voice or different sound patterns by the end of pregnancy 4
• Short-term auditory memory may be present by the end of pregnancy 4
• Sensitivity to low-frequency sounds may promote language acquisition 1
• Exposure to intense low-frequency sounds could potentially damage the developing auditory system 1

Common Pitfalls in Understanding Fetal Hearing

• Assuming that fetal movement in response to touch indicates pain perception - these are reflexive responses 3
• Confusing the ability to detect sound with the ability to process or understand it 3
• Extrapolating neonatal responses to fetuses without accounting for the different environments 3
• Overlooking the significant attenuation of high-frequency sounds by maternal tissues 5, 6