Then one of the visitors tells him that Sarah will give birth to a son within a year.
If we keep reading, we learn that Sarah is listening and her response is laughter.
Until this point in the Abraham and Sarah saga, we have only two recorded speeches by him to her. On their way to Egypt, Abraham had said, "You are so beautiful that the Egyptians will kill me in order to obtain you. Let's spare them the effort. Say you are my sister; that should protect me." Sarah did as her husband told her. The Pharaoh took her, but the Lord rescued her and returned her to Abraham (Gen 12:10-20).
The next conversation between them in when the childless Sarai tries to solve her problem by having Abram impregnate her servant Hagar. We aren't told what he said--only that he listened to his wife. When Hagar conceived, Sarai concluded that she was contemptuous of her and complained to Abram. He responded by telling her, "She's your slave; do what you want with her," (Gen 16:1-6).
In the meantime, God has spoken to Abram several times. For example, "Go. You're going to have a lot of descendants. I'm going to give land to your offspring," (Gen 12:1-7; 13:14-17; and 15:1-6). When Abram was ninety-nine God told him once again that he was to be the ancestor of a multitude of nations. He changed his name to Abraham and his wife's to Sarah, and promised that a child was to be born to Sarah (Gen 17:1-22).
The Bible is silent on how much of this promise was communicated to Sarah. When she does overhear the messenger telling her husband that she is to become pregnant, Sarah laughs. Well, why not? She's right that it's unusual for a 100-year old man and a 90-year old woman to conceive a child.
How do we react when God lays out opportunities for us? Do we let our past failures determine our future efforts?