The findings, reported in the Lancet, have led to calls for more research into the effects of ultrasound, and a leading obstetrician warns that 'prenatal ultrasound by itself can no longer be assumed to be entirely harmless'.
Ultrasound is widely used, but has never been studied through large controlled clinical trials.
A total of 1,415 women at the King Edward Memorial Hospital, Perth, were selected to undergo ultrasound at weeks 18, 24, 28, 34, and 38 of pregnancy, and another 1,419 women had one scan at week 18.
According to the report, the only difference between the two groups was significantly higher intrauterine growth restriction in the intensively scanned group.
The report concludes that 'while it is possible that this finding was a chance effect, it is also plausible that frequent exposure to ultrasound may have influenced foetal growth'.