And the LORD visited Sarah as he had said, and the LORD did unto Sarah as he had spoken. For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him. And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac.
Sarah bears Isaac in old age, after years of waiting and one bitter attempt to arrange an heir another way. The narrative gives the birth itself three flat sentences and no rejoicing; the weight of it sits in how long it took to arrive.