I know this too. As much as my mother seems disappointed when I only an average grade or even a quite good one. But pressure can be a good thing, it gives motivation to keep working.
Don't worry, they got used to seeing you having good graves and so you're expected to keep having good grades but I don't think you'll really disappoint them even if you get bad ones. They ought to enjoy seeing that you work, what could they ask more?
I think you should tell them you don't like the fact they put so much pressure on you, they'll certainly understand.