Amir believes that his and Soraya's infertility is his punishment for his betrayal of Hassan years before. In addition to the infertility affecting his relationship with Soraya, it causes Amir to question himself even more than he previously did. He tries to convince himself that it might be better that he does not have a child because if he treated his friend (Hassan) so badly, he might not be a good father. He also thinks about his own father's inability to love him the way he longed to be loved and seems to think that that might be a genetic trait that Baba passed on to him.
In the end when he conquers cowardliness and his feeling of inadequacy by rescuing/adopting Sohrab, Amir begins the process of being the best father he can be to Sohrab.
Through-out the book the relationship between Amir and his father has been a significant theme. Amir loves his father but can find no approval from him until he is an adult and in college.
Amir had spent his whole life trying to be like his father. In a way his inability to have a conceive a child reflects as another way that Amir is not able to be the man or son his father wants him to be. His father often looked at Amir as being different from him ad not living up to his expectations of him.
Baba was prolific and had been able to have two sons, but Amir can not produce one. It is only through taking on the child of his half brother, and his father's sin, that he is able to become a parent and also redeem his own soul.