yes, to check amniotic fluid levels. I have heard, someone who knows better please confirm!!, that a proper diagnosis for "low fluid" needs to be done over two u/s. So if the one comes up "low" get into bed, drink a ton of water, rest and see if it doesn't go up in a day or two. Most folks I know have had their levels go right up after that.
the other reason is to check the condition of the placenta - however from stories I'm hearing on line there are a lot of "your placenta is calcifying horribly" comments and when the baby is born, the placenta is fine. so I dn't really know what to make of u/s for checking placental condition.
BTW: 8# is not a big baby by any means. 10# might be, though not in my book. (my vba2c was 10.5#, a full pound larger than my first c/s baby, so my view of big and little are a wee skewed!)