What does Genesis 21:7 mean?

Sarah, 90 years old, continues to express her joyful response to the birth of Isaac. In the previous verse, she says that everyone who hears will laugh with her, or perhaps "at" her, since her situation is so unusual.

Now she asks, who would ever have said to Abraham that Sarah would nurse children? The implied answer is that nobody would have said such a thing. The truth, though, is that the Lord said exactly that thing to Abraham. He is the only one who could have predicted this moment, because He is the one who made it come to pass. Despite Abraham and Sarah's own doubts (Genesis 17:17; Genesis 18:12), and their schemes (Genesis 16:1–2), God has faithfully provided exactly what He said He would, in the time He said He would provide it (Genesis 18:14).

Sarah concludes her expression of joy by saying that though nobody could ever have predicted it, she has indeed borne Abraham a son in his old age.