That first ultrasound scan is an exciting time for parents-to-be, letting them see the newest member of their family for the first time and reassuring them that their baby is healthy. But could ultrasound harm the fetus? Pasko Rakic and colleagues at Yale Medical School have recently found that, in pregnant mice, prolonged exposure to ultrasound disrupts neuronal migration during fetal brain development.

It is too early to translate the team's results in mice to humans, as Rakic comments: “We do not have any evidence ourselves that ultrasound waves ... have any effect on the developing human brain” (Reuters, 7 August 2006). The conditions used in their studies were very different to those seen in the clinic. The ultrasound source was maintained in a fixed position and mice were exposed to the sound waves for 30 mins or more. When a scan is performed on expectant mothers, it is just that — a scan. The ultrasound source is continuously moved and so the fetal brain is typically exposed to ultrasound for only a few seconds.

Moreover, the changes induced by ultrasound exposure were small and it is not known whether they were significant enough to affect behaviour. The authors plan to investigate this further in monkeys.

Concerns about the effects of ultrasound on the fetal brain have been voiced before, with exposure to ultrasound being linked to decreased birth weight and delayed speech development, as well as an increased probability of being left-handed. However, the medical value of ultrasound scans is not disputed, as Rakic says: “...the benefits of the ultrasound for diagnosis are so big that I would not hesitate to use it at all” (New Scientist, 7 August 2006). Instead, in an age when ultrasound machines can be bought for use in the home, researchers suggest that expectant mothers minimize any risk by only having scans that are medically necessary.