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.
I just edited my previous post to explain myself better, i meant no way she is pregnant ready to give birth. Also her knowing she is pregnant only after some days is only viable if she wanted to get pregnant and all the factors were align that night, meaning she passed the days doing pregnancy tests to find out she was really pregnant and even in early stages, like one week or even 2, many false negatives or positives may happen, of course they may go with the route of belle doesnt know, but hades does and somehow gives her a magical pregnancy or they can go with the baby of the dark one is like this super mega powerful and making the child being born only days after the conception. Of course this is OUAT, so most likely they will follow some weird idea and maybe Belle will know she is pregnant with only 2 weeks, bc the only thing she did to pass time in SB was making pregnancy tests or she found out bc the dark one child is like magical and after rumple left she felt the baby kick, so yeah some no sense explanation, but that fans wont take in consideration much bc as long as their ship will have a baby everything is good with the story, bc lets not forget that the writers seem to forget pregnancy things related, Henry being an example, where they forgot that takes 9 months for a baby to born.
"I offended you with my opinion? Ha, you should hear the ones I keep to myself".