The latest concern comes from a study that suggests, in mice at least, that ultrasound can affect the development of the fetal brain.
Even so, researchers say the findings should not keep pregnant women from having ultrasound scans when needed for medical reasons.
When pregnant mice were exposed to ultrasound, a small number of nerve cells in the developing brains of their fetuses failed to extend correctly in the cerebral cortex.
"Our study in mice does not mean that use of ultrasound on human fetuses for appropriate diagnostic and medical purposes should be abandoned," said Dr. Pasko Rakic, lead researcher and chairman of the neurobiology department at Yale University School of Medicine.
However, he added in a telephone interview, women should avoid unnecessary ultrasound scans until more research has been done.
Dr. Joshua Copel, president-elect of the American Institute of Ultrasound Medicine, said his organization tries to discourage "entertainment" ultrasound, but considers sonograms important when there is a medical benefit.
"Anytime we're doing an ultrasound we have to think of risk versus benefit. What clinical question are we trying to answer," Copel said in a telephone interview. "It may be very important to know the exact dating of pregnancy, it's certainly helpful to know the anatomy of the fetus, but we shouldn't be holding a transducer on mom's abdomen for hours and hours and hours."
Rakic's paper said that while the effects of ultrasound in human brain development are not yet known, there are disorders thought to be the result of misplacement of brain cells during their development.
"These disorders range from mental retardation and childhood epilepsy to developmental dyslexia, autism spectrum disorders and schizophrenia," the researchers said.
Their report is in Tuesday's edition of Proceedings of the National Academy of Sciences.
