Is there any risk to the fetus with those 3D/4D ultrasounds?

A friend of mine is about 5 months pregnant and she told me about wanting to have one those 3D/4D ultrasounds done. I have heard thru word of mouth that sometimes those nurses and techs are not certified but when I looked online, some websites said there is no risk but I think that was posted by the ultrasound companies. Any one have any idea?