Gen 17:16

“וברבתי אותה AND I WILL BLESS HER — And in what did the blessing consist? In that she resumed her youthfulness, as it is said, (Genesis 18:12) “shall I have the pleasure of youth again?” (Genesis Rabbah 47:2) וברכתיה AND I WILL BLESS HER with teeming breasts. And this happened when she really needed this on the day of Isaac’s feast. For people were speaking spitefully about them, that they had adopted a foundling from the street saying, “This is our son.” Every one, therefore, brought her child with her, but did not bring the wet-nurse and she (Sarah) suckled all of them. To this refers that which is said (Genesis 21:7) “Sarah has given suck to children”. In Genesis Rabbah 47:2 this is alluded to in part.”

“And I will bless her,.... The Targum of Jonathan adds, "in her body", with fruitfulness, who before was barren, and in her soul with spiritual blessings, and in both with the blessing of eternal life: and give thee a son also of her; as he had given him one of Hagar: God had before promised Abraham a son that should be his heir, but he had not till now told him that he should be born of Sarah his wife: yea, I will bless her; which is repeated for the confirmation of it, and for the greater strengthening Abraham's faith in it: and she shall be a mother of nations; of the twelve tribes of Israel; of the two nations of Israel and Judah: kings of people shall be of her; as David, Solomon, and others, and especially the King Messiah.”

“I will bless her, etc. - Sarah certainly stands at the head of all the women of the Old Testament, on account of her extraordinary privileges. I am quite of Calmet's opinion that Sarah was a type of the blessed Virgin. St. Paul considers her a type of the New Testament and heavenly Jerusalem; and as all true believers are considered as the children of Abraham, so all faithful holy women are considered the daughters of Sarah, Gal 4:22, Gal 4:24, Gal 4:26. See also Pe1 3:6.”

“Bless, and enable her to have a son, who shall also have many children. — Whom. This is referred to Sara, in Hebrew and Chaldean; but to Isaac, in the Syriac. The blessing, at any rate, reverts to the mother; who was a figure of the blessed Virgin Mary, and of the Church; both persecuted with their children; both, in the end, triumphant. (Galatians iv. 23.) (Calmet)”