I had an u/s a couple of weeks before Flynn was born to check on the amniotic fluid levels. I don't think they do it just because tho. I had it done because they were worried I may be getting pre eclampsia.
I would try and not be too worried about how big or small your baby is going to be. The OB I saw at the hospital suggested that my baby seemed very small when he poked around my belly. Flynn ended up being a fairly average 8lbs.
You won't necessarily be like your mum and any future pgs may not be like your first! They are all very different!
HTH