When Linda enters the Central Hatchery, the workers are disgusted by her appearance. She is, to put it bluntly, fat and old. In Huxley's London, no one ever ages or gains weight, so the sight of her body is something no one can really understand. The scene only causes more humiliation and derision when John calls the Director "father". For those in the World State, this word is something to mock; no one has a father.
In contrast to the hilarity that erupts at John's words, Linda's accusation that the Director "made [her] have a baby" and that she is indeed John's mother cause a shocked silence. To the workers, what she said was the most obscene thing they could possibly comprehend. Fathers are a humorous if vulgar idea, but mothers are pornography in this society.