Komentarz do Rodzaju 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, both during her pregnancy, the act of giving birth, and in the raising of the son, all of these being painless. This is the exact reverse of the curse G'd decreed on Chavah and women in general, when He said בעצב תלדי בנים, "you will give birth to children in pain and sorrow." (Genesis 3,16) Sarah was promised to be spared that curse which other women are afflicted with. This may be the meaning of Isaiah 29,22 אשר פדה את אברהם, "who has redeemed Avraham i.e. from the painful experience associated with raising children. (Sanhedrin 19)

וברכתי, I will restore her youthfulness so that she will begin having her periods again. Another, even greater miracle, will be the fact that she will give birth. This is meant by the additional words וגם נתתי ממנה לך בן, "and I will also give you a son from her." The word נתתי is similarly used in Genesis 23,13 נתתי כסף השדה "I have given the money for the field," where also the transaction had not taken place yet, but Avraham made plain that he had already made all the arrangement s from his side. When G'd promised it is as if it had happened already.