I think it would be possible with an abdominal scan that it could obstruct, but not a vaginal scan, but I suspect it would depend on the extent of the scarring and other factors (how curvy that person is, how the uterus lies, fullness of bladder, etc.). I've had lots of first trimester exams across all my pregnancies and I've rarely had one when they could get a clear image without doing a vaginal scan. It's largely because of constipation or gas, both of which I get terribly in 1st tri. If gas can obstruct the view, I can only imagine that scar tissue might if you have enough of it. But it shouldn't be an issue with a vaginal scan obviously.