Yikes - your midwife sounds a bit keen!
First off, scans are pretty inaccurate for judging baby's weight - they can be a pound or more off either way. In any case even if your baby is 9lb that's not necessarily an issue - the actual size of the baby doesn't really matter so much as lots of factors come into play during labour, like what position you are in, what position baby is in, and so on and so forth. I have seen tiny women birth huge babies.
There is also a risk that an over enthusiastic sweep can rupture your membranes which then means that the clock is ticking then so far as getting labour going is concerned.
On the other hand if your cervix is already nice and ripe and starting to dilate then it shouoldn't hurt and can be quite succesful at triggering off labour but the thing is you'd probably be quite likely to go into labour by yourself then anyway.
If the cervix is still firm and closed (which is more than likely at 38 weeks with a first baby and no signs of labour) then doing a sweep (or putting a finger in) might be a bit uncomfortable and you might get a bit of bleeding but it is usually just ruptured blood vessels in the cervix, it might appear a bit mucousy and tinged with blood. It might trigger a few contractions but to be honest it's not likely to get you into labour by itself.
Ask your midwife to talk you through the risks/benefits before you decide what you want to do. You don't need to agree the sweep and would be quite able to ask that she waits a while. 38 weeks is not even "full term" so you still have loads of time.
Best of luck and keep us posted!