I've often been asked to remove my bottoms even at later scans as I sometimes need unexpected internals (checking cervical length, placenta position, blood flow etc - all sorts of reasons).
As a result I tend to wear a skirt (no tights) and then it is really easy. Just whip yer knickers off - takes seconds. All the places I have been to, you do this behind a curtain in the scan room and then climb onto the scan table and put a towel over yourself before calling the sonographer back through. With a skirt you have that extra layer of dignity.
An elasticated waist skirt is the very best because it's easier to pull down for abdominal scans - a particularly tight, fixed waist skirt could be more fiddly. I don't care if the sonographer sees I haven't shaved my legs in the last few days, but I care if the public does so I tend to make it a long skirt, a thick one if it's cold out because of the no tights.
The trouble with trousers is that you have nothing covering you at all then if the towel slips, plus you have the faff of taking your shoes off/putting them back on which can be pretty awful in late pregnancy! This also applies to tights, sadly.
Never, ever, wear a dress!! Fine for internals, admittedly, but just awkward and undignified for abdominals!