Ultrasound examinations during pregnancy expose the fetus to a sound as loud as that made by a subway train coming into a station, say US researchers. But doctors do not think the experience causes a baby any lasting harm.
Neither adults nor fetuses can hear ultrasound waves because they vibrate at too high a frequency for our ears to detect them. But James Greenleaf, Paul Ogburn and Mostafa Fatemi of the Mayo Foundation in Rochester, Minnesota, investigated the possibility that ultrasound could cause secondary vibrations in a woman鈥檚 uterus.
Ultrasound machines generate sound waves in pulses lasting less than one ten thousandth of a second. Pulses are used because a continuous soundwave could generate too much heat in the tissue being examined. The Mayo team predicted that the pulsing would translate into a 鈥渢apping鈥� effect.
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They listened in by placing a tiny hydrophone inside a woman鈥檚 uterus while she was undergoing an ultrasound examination. Sure enough, they picked up a hum at around the frequency of the tapping generated when the ultrasound is switched on or off. The sound was similar to the highest notes on a piano.
Theoretical consequences
When the ultrasound probe pointed right at the hydrophone, it registered 100 decibels, as loud as a subway train coming into a station. 鈥淚t鈥檚 fairly loud if the probe is aimed right at the ear of the fetus,鈥� says Greenleaf.
Fredic Frigoletto, chief of maternal fetal medicine at Massachusetts General Hospital in Boston, says doctors should be careful not to point the ultrasound probe directly at a fetus鈥檚 ear unless there is a particular reason to suspect facial or cranial abnormalities. 鈥淭hen the benefits significantly outweigh any theoretical consequences,鈥� he says.
Fatemi presented the team鈥檚 research at the annual meeting of the Acoustical Society of America in Fort Lauderdale, Florida.