Scientists are investigating whether ultrasound scans during pregnancy may harm the brains of unborn babies.

The move follows a U.S. study which showed the developing offspring of pregnant mice suffered brain abnormalities when they were exposed to ultrasound waves.

The researchers, from Yale University in New Haven, Connecticut, said the changes prompted by the ultrasounds were small and insisted that pregnant women should not stop having the scans.

Early ultrasound scans are done to determine the exact week of the pregnancy and they are also done later to check for anatomical defects and other problems.

Ultrasound waves bounce off tissue and the echoes can be used to create a 3D image of the baby.

But the waves, which are at a high enough frequency to penetrate flesh, also induce vibrations in the tissue and can lead to a rise in temperature, suggesting they could cause damage.

The team now plans further research on monkeys to see if similar effects occur in larger, more human-like brains.

If the same thing happens, there could be implications for the use of ultrasound to check on babies inside the womb.

"Those upcoming studies should give us information that will be more directly applicable to uses of USW (ultrasound waves) in humans," research leader Dr. Pasko Rakic told the Associated Press.

"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," he said.

"On the contrary, ultrasound has been shown to be very beneficial in the medical context. Instead, our study warns against its non-medical use."

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.

'Entertainment' ultrasound

The findings come just a few months after Tom Cruise bought girlfriend Katie Holmes a sonogram machine for her to use at home while she was expecting the couple's first child -- daughter Suri.

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 told AP.

"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."

In the mice study, the scientists injected more than 335 unborn mice still in their mother's wombs with special markers to track neuronal development.

Brain cells in fetal mammals multiply early and then migrate to their final destinations. Where they end up in the brain defines their connectivity and function.

Previous research has shown that when this process is upset by genetic or environmental factors such as alcohol or drugs, it can lead to mental impairment.

However, the scientists stressed that they do not know whether the changes caused by ultrasound would be large enough to alter behaviour.

The research was reported in the journal Proceedings of the National Academy of Sciences.

With files from the Associated Press