From the UK’s Health Protection Agency website:
The independent Advisory Group on Non-Ionising Radiation (AGNIR), which reports to the Health Protection Agency, has reviewed the latest scientific evidence on the health effects of ultrasound (frequencies above 20 kilohertz) and infrasound (below 20 Hz).
The report finds that the available evidence does not suggest that diagnostic ultrasound affects mortality of babies during pregnancy or soon after birth. The evidence also does not suggest any effect on childhood cancer risk. There have, however, been some unconfirmed reports suggesting possible effects on the developing nervous system – for instance, on handedness of the child.
AGNIR concluded that there is no established evidence that diagnostic levels of ultrasound are hazardous. However, further research is needed to determine whether there are any long-term adverse health effects, especially following exposure of the unborn child.