Oh bummer man, that's tough when people judge you and find you wanting according to their standards
Kaz Cooke in her book Up the Duff has advice that I'm always remembering in situations like this, which is 'it's about them, not you'.
As in, their reaction doesn't mean your pregnancy is a bad thing, but their experiences and the way they see the world affects the way they react. Perhaps your nan's known some young mums who didn't parent well and that's affected her reaction, or maybe it's whether or not you're in a r/ship or married that influences her reaction.
Do you know what I mean? She might find it too hard to move past values and prejudices she has.

Which doesn't make her treatment right, but I think once you understand why people act the way they do, it can hurt less. Because it's less personal, and more just their mindset, where they're coming from.

Would your mum or dad talk to your nan and try discover what the main issue is?
Chin up chicky, I'm sure you'll be a great mum, despite a few unenthusiastic reactions