Sasuke always thought that he was A, but he was a late bloomer and had not yet differentiated. During the Chunin Exam, under the influence of Neji's pheromones, he suddenly differentiated and turned out to be an omega! What a shame! (Naruto: "Sakura is mine!")
Because Neji is the top A, Sasuke went straight into estrus and collapsed in Neji's arms to mark him, hahahahahahahahaha (the author laughs wildly)
Sasuke: "Ah... I feel bad... Ning, Neji... Mark me quickly..."
Neji: "This... I can't bear it!" (Then I can't bear it, the author said it truthfully)
one month later
Sasuke: "Ningji... I feel pain, please, stop quickly, ah..."
Neji: "Be good, bear with me a little longer, it will be soon."
Sasuke cried, "Neji! You bastard! I'm pregnant!"
Neji: "Don't be angry, I'm still waiting for you to have a second child."
Intent: Derivative fan fiction