When will Sarah have a son according to the Lord's promise in Genesis 18?
According to the Lord's promise in Genesis 18, Sarah would have a son at the appointed time, according to the time of life, when the Lord returned. Since Abraham was then ninety-nine, the birth would occur within the next year, when Abraham would be a hundred years old at Isaac's birth.
Les explains that God's promise in Genesis 18:14 was that at the time appointed He would return according to the time of life, and Sarah would have a son. Because Abraham was already ninety-nine, the event was going to take place within the next year, since Abraham would be a hundred when Isaac was born. Les notes this was getting close to the time of conception. Sarah then backpedaled and denied laughing because she was afraid, but the Lord said, "Nay, but thou didst laugh." Les also points out that this was all within God's sovereign will, as the name Isaac means laughter, making him a child of laughter even before conception.
Is any thing too hard for the LORD? At the time appointed I will return unto thee, according to the time of life, and Sarah shall have a son.