Charles Ellicott Commentary Genesis 21:6-7

"And Sarah said, God hath made me to laugh. Every one that heareth will laugh with me. And she said, Who would have said unto Abraham, that Sarah should give children suck? For I have borne him a son in his old age." — Genesis 21:6-7 (ASV)

God has made me to laugh. —Sarah’s laugh was one of mingled emotions. Joy was uppermost in her mind, but women do not laugh for joy at the birth of a child. Doubtless, she recalled the feelings with which she listened to the announcement that she would bear a son, an announcement made by those whom she then regarded as mere passing wayfarers (Genesis 18:12), but whom she had long since known to be the messengers of God.

And still the event seemed marvelous and astonishing to her, so that, as she said, all that hear will laugh with me—in Hebrew, for me, or over me—not “will ridicule me,” but will be merry at the thought of an old woman of ninety having a son. Deeper feelings would come afterwards, and the acknowledgment that what was contrary to nature was worked by Him whom nature must obey; but surprise is uppermost in the little poem in which Sarah expresses her first feelings:—

Who would have said to Abraham
Sarah nurses sons?
For I have borne a son to his old age.