Maybe there are rival magical beings from outside the Beast's domain. They see the potential of the baby that Casey is carrying and want to either take it for their own purposes or stop it from being born because they fear it will grow up to be a powerful force against them.
It could be the jealous servants in the Beast's castle. They might not like Casey in the first place, and her pregnancy could make them fear that she will gain even more power and influence. So, they could try to sabotage her pregnancy in secret ways.
The Capitol could still be the main antagonists. They might see Katniss's pregnancy as a threat, either because they fear the child could grow up to be a symbol of resistance like Katniss or just because they want to maintain control over her. They could try to harm her or the baby in some way.
Some of the more traditional pure - blood families in the wizarding world could be antagonists. They might not approve of Harry, a half - blood, being involved with Pansy, and they could try to stop the pregnancy or break up the couple for the sake of 'pure - blood' ideals.
Another possibility is a group or organization that has different ideals. If Thalia and Luke are part of a group that values something different from them, that group could oppose their decision to have a child and become the main antagonists, trying to stop the pregnancy or take the baby away once it's born.
Well, it could be certain political rivals. Maybe there are those who fear that the child of Korra and Iroh will have a strong claim to leadership or influence in the future. So, they become the antagonists, trying to create problems for the couple during the pregnancy. It could also be some dark spirits or entities that sense the special nature of the unborn child and want to prevent it from being born, seeing it as a potential obstacle to their own dark purposes.
Voldemort and his Death Eaters could be the main antagonists. Since they are against the wizarding world's light side, they would see Lily and James's unborn child as a threat, especially if there are prophecies surrounding the baby.
It could be Sharpay's over - controlling mother who is more concerned about her daughter's image and career than the pregnancy. She might try to convince Sharpay to take certain actions that are not in the best interest of the baby or Zeke.
Lex Luthor could be a great antagonist. He might see the unborn children as a threat to his power and plans for world domination. He could try to steal the babies' powers while they're still in the womb or manipulate events to cause harm to Barry and Kara during their pregnancies.
Voldemort and his Death Eaters could be the main antagonists. They might see the unborn child as a threat and try to harm Lily or the baby. They could also be after some power that they believe the child will possess.
The AST (Anti - Spirit Team) could be antagonists if they misunderstand the situation. They might think that the unborn child could be a new threat and try to stop Tohka from having the baby. Their lack of understanding of the spirits' true nature could drive them to take actions against Shido and Tohka.
It could be an ex - partner of either Leah or Jasper. For instance, if Jasper has an ex - girlfriend who is jealous of Leah and the new baby, she might try to cause trouble between them. Maybe she spreads false rumors about Leah to make Jasper doubt her.