When the ultrasound is directed directly into the fetus’s ear canal, the noise level is claimed to be comparable to that of a subway train. In addition, the amniotic fluid is supposed to be warmed by the ultrasound because of the vibrations it experiences. But is that really the case? Are these screening check-ups safe for expecting mothers?
“Yes”, at this time, since there is no proof that ultrasound exams are harmful to the developing fetus. But this should still only be done when absolutely required and by physicians who have received proper training and have further educated themselves in the area. Experts advise against using ultrasound for the sole purpose of “baby television” since precaution should always be a top priority in medicine. It’s important to be conservative with the use of any diagnostic tools at our disposal.
The unborn infant has no defenses against the heat
But what do we know about the effects of noise and heat on newborns? Ultrasound is a sound wave that causes mechanical effects and temperature increases in the tissues it passes through. When the material starts to vibrate, it generates heat. Heating the surroundings of the unborn child is dangerous since they cannot yet control their own body temperature. When it becomes too hot, this might result in developmental damage. It would be quite concerning if ultrasonography caused the amniotic fluid to warm to a detectable degree.
Because animal studies suggest that regular ultrasonography examinations can actually increase tissue temperature by roughly 1.8 degrees Fahrenheit (1 degree Celsius). According to these studies, after several minutes of using pulsed Doppler ultrasonography, temperatures might rise as high as 7 degrees Fahrenheit (4°C).
Doppler ultrasound, however, is used only for prenatal care examinations of the fetus’s heart and blood vessels. A few seconds are all that are needed to do this check-up. This means that a significant rise in temperature from ultrasound scan is quite improbable, and the baby is pretty much safe.
But, each tissue and material still generate different amounts of heat. In this sense, ultrasound wavelengths are weakly absorbed by amniotic fluid. It’s also tough to heat up, which makes sense. Neither conventional sonography nor Doppler ultrasonography showed any evidence of a rise in amniotic fluid temperature in the one research study that evaluated this possibility.
Ultrasound is not audible by humans
Therefore, it’s quite unlikely that an unborn child can hear the ultrasound in the womb. Around 5–10 MHz (or 5–10 million oscillations per second) is the frequency range used in diagnostic ultrasonography. However, the human ear can detect vibrations at a rate at least 50 times lower than that—20.000 vibrations per second.
During the ultrasound examination, the ultrasound is delivered not in a continuous stream, but rather in a series of brief, quickly spaced bursts. However, some doctors believe that this so-called pulse repetition rate might potentially be heard by the baby, and the unborn child may interpret this as a high-pitched sound.
To date, however, there is no data published in peer-reviewed, scholarly publications to support the claims that the fetus can hear, respond to, or be injured by this pulse repetition rate. Therefore, there is no reason to avoid preventive ultrasound examinations or to worry about possible harm to the unborn child.