Sengoku reveals that Portgas D. Rouge kept Ace in her womb for 20 months to protect him from the World Government.
This episode is widely considered the "calm before the storm," focusing on setting the massive scale of the upcoming war. One Piece (Dub) Episode 459
If you want to see how the community reacted to these massive plot twists, these creators captured the hype perfectly: Sengoku reveals that Portgas D