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.
The main antagonists could be some of the more traditional pure - blood families who look down on Ron and Luna's relationship. They might see Luna as an oddity and Ron as not being from a high - enough status family. They could try to interfere with the pregnancy, either by spreading false rumors or trying to separate the couple.
It could be the traditional enemies from Naruto's world, like the Akatsuki. They might see the union of Naruto and Rias and their unborn child as a threat to their plans for world domination. They could try to kidnap Rias to use her as a bargaining chip against Naruto or to study the unique nature of their child.
The old enemies of Percy and Annabeth like the Titans or some of the more vengeful monsters could be the main antagonists. They might see the unborn child as a threat or an opportunity to get back at Percy and Annabeth.
Their friends could also be antagonists in a sense. For example, if there's a friend who has always had a crush on Freddie and is jealous of Sam. This friend could spread rumors about Sam being a bad influence or try to convince Freddie that he's making a huge mistake by staying with Sam and having a baby. Another scenario could be that their friends don't take the situation seriously and keep pressuring them to act like they used to, which makes it hard for Freddie and Sam to focus on the pregnancy.
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.