What is the significance of the two B'rachos to Sarah mentioned in this Pasuk, one before giving birth and one afterwards?
Rashi: Hashem blessed her a. that, in spite of her being ninety years old, she would regain her youth; 1 b. that the capacity for milk would return to her breasts, to feed not only her own baby, but many others too. 2
Targum Yonasan: The Berachos that Hashem promised Avraham (in 17:6) would be fulfilled through her.
Seforno: a. Hashem blessed her that she would bear a son; b. she would merit an easy pregnancy and birth, and she would rear her son without pain. 3
See Rashi to 18:8.
QUESTIONS ON RASHI
Rashi writes: "'And I shall bless her' - regarding nursing. She needed this blessing on the day of Yitzchak's feast... she nursed all [of the babies]." How do we know that this blessing refers to nursing?
Gur Aryeh: The beginning of the verse already stated, "I shall bless her;" so this additional blessing must refer to nursing. It cannot refer to nursing her own child, for any woman who gives birth is able to do so. The blessing must refer to the ability to nurse many children.
Rashi writes: ":And I shall bless her - ... On the day of Yitzchak's feast... she nursed all [of the babies]." A blessing is supposed to be of benefit to the recipient; of what use was this blessing to Sarah herself?
Gur Aryeh: The essence of a blessing is to tap into the Source of all blessing [Who grants abundance in all areas]. Sarah was [already] granted milk that would flow like a fountain, but this only became apparent on the day that she had to nurse many babies.
Rashi writes: "[The invited women] did not bring the babies' nursemaids along." Why did they act in this way?
Gur Aryeh: Each woman planned to test Sarah by asking her to nurse her baby. [Many people would mock Avraham and Sarah, saying that they had brought home a foundling from the market, calling him their own son. Sarah's ability to nurse would prove that she herself had given birth to Yitzchak.]