Hold on, are we talking about fanfiction tropes or actual canonical plot points? Because I've read a mountain of stories, especially on Archive of Our Own, and Hermione being pregnant is rarely a twist about the pregnancy itself. It's a catalyst. The popular 'twist' I see over and over is the secret pregnancy kept from the father due to a magical separation—like Ron being obliviated after a fight and Hermione's magic hides the pregnancy, or a life debt to Draco that makes her flee with his child. The twist isn't 'oh she's pregnant'—it's 'oh, and the father is him, and here's the cursed reason he never knew.' The reveal usually involves accidental magic from the toddler giving it away.
Honestly, a lot of these plots feel like elaborate setups for a Dramione or Snamione reconciliation scene. There's a whole sub-genre where the 'twist' is the magical lineage of the child revealing a soulmate bond or breaking an ancient curse, which feels like an excuse to get the estranged couple in the same room. It's pure wish-fulfillment, but sometimes you're just in the mood for that complicated, angsty reunion with a baby as the bridge.