Thanks for the information Mikey.
I was curious about this too, so am pleased to hear a scientific analysis.
My obs here said that there is no harm, but like Canada, in Australia where I am from 2 scans are the normal.
I do believe this comes down to cost, not many obs there would have the machine in their office, there you would go to a deperate clinic for your scan.
Having had a miscarrige in early pregnancy in the past, I think it is those early scans that are the most useful. It would be horrible to get to the 12 week ultra sound, as people I know there have, only to find then that your baby has no heart beat, and had stopped developing previously. I think we are pretty lucky to have the opportunity to check the development so often, it is certainly good for peace of mind.
The internal method, is because the pregnancy sac is not readable that early from your abdomen. I wouldn't think that would be any more harmful, as it is not actually interfering with your uterus in any way.