Sarah conceived — Sarah by faith, received strength to conceive, Hebrews 11:11. God therefore, by promise, gave that strength. Abraham was old, and Sarah old, and both as good as dead, and then the word of God took place.
Related Commentaries of Genesis 21:2
Genesis 21:2
2 For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him.