It's impossible to know whether or not she was born in the United Realms because there's no clear indication when the United Realms were created. We also don't know exactly how much time passed between Henry leaving Storybrooke and the curse being cast and how much of it was realms weirdness, so we don't know when Emma got pregnant. Any attempt to figure it out would be just as speculative as claiming she was from Storybrooke... so honestly, I'd just say she's from Storybrooke. That's where Emma and Hook live as far as we're aware, and I doubt Emma would stray far from modern medicine to give birth. Even if the realms were united by the time Hope was born, it was probably still within the boundaries of Storybrooke.