I had that but by the end of the pregnancy it had absorbed back in (or something!) and I didn't have the problem anymore. I was quite worried at the time as I was given the impression it could mean being induced early and all sorts of things but no-one seemed to tell me it could just go away on its own!
Ask your midwife to explain all the possible out comes of it and put your mind at rest.
A friend had a lot of extra fluid (like two litres instead of one) - although they couldn't really tell how much until she was in labour and her waters broke. They can't tell how big the baby is so they can't really tell how much fluid is in there either. Lots of guesswork going on!
The only difference it made to my friend was that she was huge and they hassled her about her baby being too big (which she wasn't).
If you have lots of extra fluid its called Polyhydramnios but it doesn't sound as if you do.