Search the internet and there are lots of strange and interesting stories about ultrasound being dangerous to baby’s hearing or that the baby has its hands by the ears to shield itself from the ‘noise.’ Diagnostic ultrasound images are made by sending VERY high frequency sound waves into the uterus at frequencies above 3 million herz. Humans can only hear sounds with wavelengths between 20 and 20,000 herz. Frequencies above that are simply impossible to hear, and exposure to them will not do any damage to you or your developing baby. Sometimes, you can hear the ultrasound probe give an audible hum. This is only reverberations or scatter of lower-range frequencies that are by-products of the range of frequencies emitted. It is probable that the fetus hears that hum as well since they are able to hear things after 14 to 16 weeks.
So, what can the baby hear?
Higher pitched sounds do not travel as well into the uterine as lower, bass-type sounds. To get an idea of how your fetus is hearing the world outside, try lying with your head underwater in a bathtub and listen to music on a nearby radio. You will hear the lower-pitched sounds and the drums best. Keep in mind that the porcelain of the tub itself will transmit sounds much better than the soft tissues of a mother’s body. Additionally, outside sounds compete with the sounds going on inside the mother, most prominent of which would be the mother’s heartbeat and her own voice. Many studies show that right from birth, newborns are able to recognize and show preference for their own mother’s voice, turning their head towards her voice over that of strangers.
Why does my baby have his/her hands by its head and face at every ultrasound?
Parents often comment that their little one likes to have its hands in front of the face or by the ears on every scan. Fact is, most babies do this. They are not doing it to shield themselves from noise or light (and NO, the probe does NOT emit any light into the uterus which is a question I’ve gotten surprisingly often). They are floating in a fluid-filled environment and it is a natural position for the hands to be up by the head. Try immersing yourself in a pool, exhale so you sink underwater, and relax as best you can. You too will find that your elbows will want to bend and your hands will float upward towards your face.