Ultrasound scans damage the brains of mouse embryos, according to a study aimed at mimicking the effects of pregnancy scans in humans.

The changes prompted by ultrasound are small, the authors said, adding that pregnant women should not stop having the scans.

They are planning tests in monkeys, but if the same thing happens there could be implications for the use of the technique to check on babies inside the womb.

Ultrasound waves are at high enough frequency to penetrate flesh. The waves bounce off tissue and the echoes can be used to create a 3D image of the baby.

But the waves also induce vibrations in the tissue and can lead to a rise in temperature, suggesting they could cause damage.

There is tentative evidence that frequent exposure to ultrasound in the womb can lead to a decrease in body weight at birth, an increase in left-handedness and delayed speech development.

Pasko Rakic and his team at Yale Medical School in New Haven, Connecticut, have shown that the brains of mouse embryos exposed to ultrasound develop abnormally, although they stress that they do not know whether the changes would be large enough to alter behaviour.

"I want to emphasise that our study in mice does not mean that use of ultrasound on human foetuses for appropriate diagnostic and medical purposes should be abandoned," he said.