Do I really have to... seriously?

I have my scan tomorrow and they said to drink 500mls an hour before. Surely the baby is big enough to see now without needing to do this? I can't do it, I only drank about that much for the 12 wk scan and it was painful. I feel like I constantly have a full bladder atm anyway so I absolutely won't be able to do it. So, if I don't, is there going to be a problem?