Cost and effectiveness
Many parents ask whether the use of ultrasound is safe during pregnancy. The answer is, at this stage we think so, but we cannot be sure because there is insufficient research to confirm it. The following is an explanation of the known physical effects of ultrasound and how caregivers try to minimise them when using this type of technology on pregnant women and unborn babies.
Ultrasound heats body tissues and has been shown to create temperature changes of up to 40-45o Celsius at 1MHz, to a depth of 5cm. The higher the ultrasound frequency, the faster the tissue can heat up, with 3MHz heating tissue 3 times faster than a 1MHz, but to less depth. The amount of temperature rise depends on the type of body tissue absorbing the ultrasound waves, with the highest absorption being into bone and the lowest absorption into amniotic fluid, with varying levels of absorption into other body tissues.
This physical 'heating' effect is the basis of therapies using ultrasound for treating injured muscles and tissues (as well as other medical applications), known as 'therapeutic ultrasound'. However, therapeutic ultrasound generally involves continuous ultrasound waves and long 'dwell times' on one area of the body. Diagnostic ultrasounds (for creating images during pregnancy) are intermittently 'pulsed' with the aim of reducing heating of the body tissues and hopefully making ultrasound safer for the unborn baby. Vaginal ultrasounds tend to use higher frequencies so they do not travel as deep, because the transducer is closer to the baby, but are potentially more likely to heat the baby or surrounding tissues.