Then the Lord took note of Sarah as He had said, and the Lord did for Sarah as He had promised. So Sarah conceived and bore a son to Abraham in his old age, at the appointed time of which God had spoken to him. Abraham named his son who was born to him, the son whom Sarah bore to him, Isaac.
Genesis 21:1-3 (NASB)
Back in Genesis 12, God made a promise to Abraham, but a careful reader will note that the birth of Isaac is not Abraham’s miracle. He can sire children, as we see with Ishmael.
This is Sarah’s miracle. She was barren.
Genesis 21 puts the spotlight squarely on her.