But why never never changed sex during the 5 years without a baby? it's definitely a plot flaw that the author forgot to clarify but there are two possibilities to make some sense of it:
1.- The interruption of any process related to the curse is forever (the most logical thing).
2.- It is temporary and Chroma is pregnant again.
Let me explain.
Perhaps the curse is not made to affect another living being that is not the originally cursed one (the heroes), even any process related to the curse can be interrupted if an unplanned third party is affected (either the healing or its creation).
I mean, if Chroma were to change her sex to a man by undoing her curse, the change in her genitalia would kill the baby in her womb, a living being not originally contemplated in the curse (this is directed at the heroes specifically, not their children) so, the process of getting rid of the curse is interrupted (indefinitely or even forever).
But as I said, maybe is interrupted forever to make some sense.