“Is ultrasound safe for my baby?” and “I know someone whose baby was born too small because of all the ultrasound she received during her pregnancy”. These are two sentences that you might hear during your busy day in the ultrasound unit. The AIUM Official Statement “Conclusions Regarding Epidemiology for Obstetric Ultrasound” states: “Based on the available epidemiologic data, there is insufficient justification to warrant conclusion of a causal relationship between diagnostic ultrasound and recognized adverse effects in humans. The epidemiologic evidence is based primarily on exposure conditions existing prior to 1992, the year in which maximum recommended levels of acoustic output for ultrasound machines were substantially increased for fetal/obstetric applications. Some older studies have reported effects of exposure to diagnostic ultrasound during pregnancy, such as low birth weight, delayed speech, dyslexia, and non-right-handedness. Other more recent studies have not demonstrated such effects. The absence of definitive epidemiologic evidence does not preclude the possibility of adverse effects of ultrasound in humans.”
Why is this statement important to all practitioners of ObGyn ultrasound?
Because knowing the information will enable you to answer patient questions and comments mentioned at the beginning of this post. What the AIUM statement explains is that studies performed on specific large human populations, with defined methods did not show that diagnostic ultrasound is responsible for harm in humans. (Studies such as this are what epidemiology does: examine how often diseases occur in different groups of people and why.)
While in the past, there were some publications that suggested some effects, such as low birth weight, more controlled studies have not been able to demonstrate such effects in humans. An important point is that many studies are relatively old and were performed before maximum recommended output of ultrasound machines meant for OB use was increased from 94mW/cm2 to 720mW/cm2. This increase was intended to obtain more detailed images. The US Food and Drug Administration (USFDA) agreed with ultrasound instrument manufacturers’ requests to allow this increase, on the condition that two numbers were displayed in real-time on the monitor of the ultrasound system:
- The thermal index (TI) to show the possibility of increased temperature, secondary to energy absorption by the tissues, and
- The mechanical index (MI) to convey the risk of direct effects of the sound waves.
If these are kept low, no noxious effects are demonstrable, as expressed in the Epidemiology statement. This includes physical as well as mental effects. What are low indices? If the TI is <1 (the scientific number is 0.7, but 1 is easier to remember), there appears to be no risk of thermal effects for exposure under 1 hour. Regarding non-thermal or mechanical effects, based on the absence of gas bubbles in the fetal lungs and bowels (the two organs where effects were shown in animals after birth), no effects are expected in human fetuses. Demonstrating long-term effects or lack thereof, particularly if subtle, is much more complicated.
The statements issued by the AIUM’s Bioeffects Committee are intended as baseline considerations in practice. As ultrasound continues to be adopted into clinical use, the Bioeffects Committee will continue to monitor outcomes in order to inform and educate the community.
Jacques S. Abramowicz, MD, is a professor in the Department of Obstetrics and Gynecology at the University of Chicago.
Interested in learning more about the bioeffects of ultrasound? Check out the following AIUM Official Statements:
- As Low As Reasonably Achievable (ALARA) Principle
- Statement on Biological Effects of Ultrasound in Vivo
- Prudent Use and Safety of Diagnostic Ultrasound in Pregnancy
- Recommended Maximum Scanning Times for Displayed Thermal Index (TI) Values
Abramowicz JS, Fowlkes JB, Stratmeyer ME, Ziskin MC. Bioeffects and Safety of Fetal Ultrasound Exposure: Why do we Need Epidemiology? In: Sheiner E, (ed.): Textbook of Epidemiology in Perinatology. New York: Nova Science Publishers, Inc.; 2010.