Regina could have very well purposefully made herself unable to conceive to prevent herself from ever getting pregnant by Leopold and the Huntsman! A simple curse that can be undone…
We’ve never heard of Regina using such a spell, so that is pure speculation. All we have to go on is that Regina lived in a pre-industrialized society in which babies were born frequently because birth control seems not to have existed. Regina was having sex with at least two different guys so her not conceiving a child suggests she’s barren, rather than either Leo or the Huntsman being sterile.
For lots of reasons, I think Belle getting pregnant would be better than Regina (at least at this stage). Until Robin sorts things out with Marian and their marriage is shown definitively to be over, I hope for Regina’s sake she’s not pregnant by him. If Robin weren’t already spoken for and didn’t have a child with another woman, I’d be in favor of that too. But until I see a formal divorce paper, I hope there is no child for Robin and Regina. I know Lily being their child is a popular theory, but until we get firm evidence that is the case, I’m not yet willing to bet on it.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy