Sarah To Bear Her Own Son

Genesis 17:15-22​
 
15  And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be. 
16  And I will bless her, and give thee a son also of her: yea, I will bless her, and she shall be a mother of nations; kings of people shall be of her. 
17  Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear? 
18  And Abraham said unto God, O that Ishmael might live before thee! 
19  And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him. 
20  And as for Ishmael, I have heard thee: Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation. 
21  But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year. 
22  And he left off talking with him, and God went up from Abraham.