Sforno on Genesis 18:12/ספורנו על בראשית י״ח:י״ב

1ותצחק שרה, she assumed that the words of the angel were merely a blessing, not a prophecy, as occurred with the prophet Elisha. (Kings II 4,16) She believed that when such a blessing is given to old people, people who would require a miracle for the blessing to come true, it would not be effective in her case. She considered the kind of rejuvenation required for her to bear a child as equivalent to reviving the dead. Such an act, in her opinion, required G’d’s personal attention, not merely that of a prophet. At the very least it would come about in answer to a prayer.