this is a web site wher I found really scary stuff on ultrasound:
just a fragment of the article:
Ultrasound scans, routinely used to look at internal organs and to monitor the growth of a developing fetus, can stop cells from dividing normally, Irish scientists said Wednesday.
Researchers at University College in Dublin told New Scientist magazine it is too early to tell if the changes they found in the cells of mice are harmless or what the implications of the findings could be for humans.
"It has been assumed for a long time that ultrasound has no effect on cells," said Patrick Brennan, who led the research team. "We now have grounds to question that assumption."
During the study, the rate of cell division in mice that were given an 8 megahertz scan lasting 15 minutes was 22 percent lower than normal, and the rate of cell death doubled.
Routine hospital scans use frequencies between 3 and 10 megahertz and can last up to 60 minutes.
Brennan said the sound waves of the scans could be damaging the DNA in cells, which could delay cell division and repair, or it might be switching on p53, a tumor suppressor gene that controls cell death.
for more reffer to the website, there is more info about this.