Worry less about upsetting your friend, and more about your own wellbeing and your babies.
Going to maybe stir some things here, but weddings are just not that important. They're made up things.
However, you cannot predict how your health will be. You cannot predict how your babies health will be.
Your partner wasn't invited to the wedding seems odd.
I don't know much about babies but I'm not sure if having a potentially very young baby around many many people could be bad for their health? I'm not sure if there's anything about their immune system.
In short, take care of yourself.
Its sweet you care about your friends feelings.
But, she also has put you in an awkward position where she hasn't invited your partner. And expects you to go without baby/just after around when you give birth.
If I were you, I would say no. Say you'd love to go, but you don't know how things will be around then. And it's better you say no so she doesn't factor in your costs/seat/outfit ect.
You could have a nice chat about it.
Say you just don't think it's likely to be feasible, but much much closer to the date, if it does seem possible you would consider popping in. But your not sure what that would mean in terms of time ect.
Maybe you'd say you'd maybe need your partners support.
Essentially say no thanks. But if she really insists that she wants you there, then say you would need to have a conversation around a week before the date so you know where you're at. If she's not willing/able to be flexible in that regard, and also not consider potentially having your partner there for an hour or something. Just give an all out no.