the time doesnt pass diferent in SB and in UW, so her being pregnant can only happen if she gets a magical pregnacy like Zelena.
No, it’s still possible that she can be pregnant. It was Zelena being able to give birth itself that was magically sped up. It would only take up to roughly 2 weeks at the most for Belle to learn, and I guess they really could be in the Underworld for that long.