David learns something important about his society when Aunt Harriet arrives with her baby.  David is aware that no one with any kind of imperfection is allowed, but he never considered what that might mean until his aunt came begging his mother to borrow her new baby.

 'She was born a week ago. I didn't know what to do. Then when I heard your baby had come early and was a girl, too, it was like God answering a prayer.' (ch 7)

Harriet wants to swap babies long enough to get a certificate for hers.  Both babies are girls.  Unfortunately, Harriet’s has some little thing wrong with her that prevents her from being perfect.  Since she is not perfect, Harriet cannot keep her.  She will be taken away—like Harriet’s two previous children.

I can't stand that -- not again. Henry will turn me out, I think. He'll find another wife, who can give him proper children. There'll be nothing -- nothing in the world for me -- nothing. (ch 7)

Harriet is worried that she will lose her husband as well as her baby, because she has had three children with imperfections.  David’s mother refuses to help.  She calls her husband, who tells Harriet to turn the baby in and repent.  Harriet has no choice.  If she does not turn the baby in, they will. 

Harriet’s body is found.  She killed herself.  No one mentions what happened to the baby.

David is clearly struck by this event, because he remembers it when the Sealanders come to rescue him.  It becomes symbolic of everything that is wrong with his community.  That an innocent baby should suffer because of one small imperfection is baffling and sickening to him.  He is happy to be moving on.