Study reveals possible dangers of prolonged fetal ultrasound exposure
A new study undertaken by Yale researchers has found that prolonged and frequent use of ultrasound on pregnant mice causes brain abnormalities in the developing mouse fetus, raising concerns that the same could be true for human fetuses. The researchers, from Yale’s School of Medicine, reported their findings in the Proceedings of the National Academy of Sciences.
"Proper migration of neurons during development is essential for normal development of the cerebral cortex and its function," said Pasko Rakic, MD, chair of the Department of Neurobiology and senior author of the study. "We have observed that a small but significant number of neurons in the mouse embryonic brain do not migrate to their proper positions in the cerebral cortex following prolonged and frequent exposure to ultrasound."
The study is believed to be the first to look at the impact ultrasound waves (USW) appear to have on neuronal migration in mice brains at a late stage of embryonic development, a period during which the fetus is most vulnerable.
To gather their findings, the researchers injected over 335 fetal mice at embryonic day 16 with special markers to track neuronal development. Exposure to USW for 30 minutes or longer caused a small but statistically significant number of neurons to remain scattered within inappropriate cortical layers and/or in the adjacent white matter, a release of the findings states.
"These findings suggested the desirability of further work in this area. We do not have any evidence ourselves that USW cause behavioral effects in mice or have any effect on the developing human brain," Rakic said.           
"I want to emphasize that our study in mice does not mean that use of ultrasound on human fetuses for appropriate diagnostic and medical purposes should be abandoned. On the contrary: ultrasound has been shown to be very beneficial in the medical context. Instead, our study warns against its non-medical use,” added Rakic. “We intend to conduct further research, which will focus on non-human primates, to see if a similar effect is occurring in the developing larger brains, which are more similar to humans. Those upcoming studies should give us information that will be more directly applicable to uses of USW in humans."

In response to concerns that have been raised due to the release of this study, the American Institute of Ultrasound in Medicine (AIUM) has sought to reassure the medical community and patients that diagnostic ultrasound, if practices properly, is perfectly safe. This is because, the organization states in a release, the whole brain exposure to the developing mouse brain described in the study “differs significantly from the short duration of diagnostic ultrasound imaging to selected sites in the human fetus.”
AIUM Bioeffects Committee Chair Jacques S. Abramowicz, MD, states that, “there is nothing to show that ultrasound is in any way harmful to the human fetus if the test is done when medically indicated by people who know what they are doing, but exposures should be kept to the lowest levels needed to achieve the medical aims.” 

As a precaution, AIUM is encouraging patients to check that their practitioners have received specific training in fetal ultrasound imaging so that the best results are achieved during exams.
This study was supported by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health.