James Greenleaf, Paul Ogburn and Mostafa Fatemi of the Mayo Foundation in Rochester, Minnesota have found that ultrasound examinations during pregnancy expose the fetus to a sound as loud as a subway train coming into a station. But they don?t think the experience causes the baby any lasting harm.
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.
Neither adults nor fetuses can hear the actual ultrasound waves because they vibrate at too high a frequency for our ears to detect them. But researchers found that ultrasound causes secondary vibrations in a woman?s uterus.
The researchers listened by placing a tiny hydrophone inside a woman?s uterus while she was undergoing an ultrasound examination. They picked up a hum at around the frequency of the tapping that is generated between pulses, when the ultrasound is switched on or off. The sound was similar to the highest notes on a piano. When the ultrasound probe pointed right at the hydrophone, it registered 100 decibels, as loud as a subway train coming into a station. ?It?s fairly loud if the probe is aimed right at the ear of the fetus,? says Greenleaf.
Fredic Frigoletto, chief of maternal fetal medicine at Brigham and Women?s Hospital in Boston, says doctors should be careful not to point the ultrasound probe directly at a fetus?s ear unless there is a particular reason to suspect facial or cranial abnormalities. ?Then the benefits significantly outweigh any theoretical consequences,? he says.
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