Ultrasound has a demonstrated record of safety for more than 50 years of clinical use. Although no independently replicated epidemiologic data exist to suggest harmful effects of ultrasonography in the fetus, ultrasonography is a form of energy with two main bioeffects in tissue: heat, a direct effect, and oscillatory movements, secondary to the alternating positive and negative pressure waves. These effects are inherent in the physical properties of ultrasonography and have not been shown to be harmful in humans.30
However, most safety data are epidemiologic, collected before the permissible output of scanners was increased by a factor of almost 8, around 1992.31
The U.S. Food and Drug Administration recommends against the use of medically unindicated or commercial prenatal ultrasonography.32
Although there is no evidence of harm, ultrasound power levels have gone up, and there is increasing use of more powerful color and spectral Doppler in the first trimester, so safety cannot be presumed. Dose is a quantitative measure that combines intensity and exposure time. No standard dose quantity has been identified for ultrasonography. Variation in tissue properties between individuals and scanning conditions influence dose in unpredictable ways. For all practical purposes, fetal dose cannot be precisely quantified. Documentation of dwell time, type of machine, and transducer used would begin to address the problem of lack of a dose metric for ultrasonography.
The bioeffects of clinical ultrasound exams are approximated by the thermal index for heating and by the mechanical index for cavitation effects. Both are indices of exposure, but neither takes time into account. Thermal index predicts potential for temperature increase, not actual rise, and it remains unknown whether there is a threshold for temperature-related bioeffects. Mechanical index expresses potential to induce inertial cavitation. Mechanical effects are less likely in the fetus, because foci susceptible to cavitation (ie, containing gas) are not present.30
Maternal body size is important because examinations may be prolonged in heavier women, whereas at the same time fetal exposure may be reduced per unit time due to attenuation of the acoustic beam at greater depths.
Evidence in animals shows that exposure to diagnostic ultrasonography can produce significant temperature increases in the fetal brain near bone. The critical questions include whether the extent of ultrasound-induced temperature rise is sufficient to create a hazard and whether there is a threshold for hyperthermia-induced birth defects. In addition, the determination of possible neurophysiologic effects or responses to clinically relevant exposures is needed. In a review of epidemiologic studies of human exposure to ultrasonography, there were no effects noted on childhood cancer, dyslexia, speech development, or congenital anomalies.33
However, there is very limited evidence that the frequent exposure of the human fetus to ultrasound waves may be associated with a nonsignificant decrease in newborn body weight,34
a reduction in the frequency of right-handedness,35
and delayed speech.36
Ang et al37
examined the effect of ultrasound waves on neuronal position within the embryonic cerebral cortex in mice. When mice were exposed to ultrasound waves for a total of 30 minutes or longer during neuronal migration, a small but statistically significant number of neurons failed to acquire their proper position and remained scattered within inappropriate cortical layers and/or in the subjacent white matter. The magnitude of dispersion of labeled neurons systematically increased with duration of exposure to ultrasound waves. The relevance of these findings for cortical development in humans is unclear. The ultrasound beam characteristics used in this study were well within clinical norms for fetal exams. There are, however, significant differences in the number of neurons and the size of the cerebral cortex between mouse and human. The distance between the exposed cells and transducer in Ang’s experiments was shorter than in humans. Furthermore, the duration of neuronal production and the migratory phase of cortical neurons last 18 times longer in the human fetus than in mice. Thus, an exposure of 30 minutes represents a much smaller proportion of the time dedicated to development of the cerebral cortex in humans than in mice, making human corticogenesis less vulnerable to ultrasound waves.37
All these questions raise important future research issues.
Continuing research into bioeffects and safety of ultrasonography in pregnancy is warranted. It is essential to examine the possible effects of ultrasound waves on cortical development in nonhuman primates, where the duration of embryogenesis and the size and complexity of migratory pathways are similar to those in humans, as well as to perform comprehensive epidemiologic studies in humans.