Why did it take a miracle of God for Sarah to conceive Isaac?
Sarah was almost 90 and Abraham almost 100, so conception was naturally impossible; it required God's miraculous intervention. That miracle fulfilled God's promise that at this time He would come and Sarah would have a son, preserving the promised seed through Isaac rather than Ishmael.
Les explains that Abraham was almost 100 and Sarah was almost 90, so it had to take a miracle of God for her to conceive. That miracle was the result of the promise of God, not human ability. God said, “At this time will I come, and Sarah shall have a son.” Les points out that these promises go all the way back to eternity past, before Adam was created, and were already set in God's concrete. The seed of the woman, singular, is Christ; the seed could not come through Ishmael or Esau, but had to come through Isaac. Romans 9:9 cites this word of promise, and Les emphasizes God's sovereignty in it.
For this is the word of promise, At this time will I come, and Sarah shall have a son.