Adam Clarke on Genesis 18:12

Sarah laughed - Partly through pleasure at the bare idea of the possibility of the thing, and partly from a conviction that it was extremely improbable. She appears to have been in the same spirit, and to have had the same feelings of those who, unexpectedly hearing of something of great consequence to themselves, smile and say, "The news is too good to be true;", see Gen 21:6. There is a case very similar to this mentioned Psa 126:1, Psa 126:2. On Abraham's laughing, See note on Gen 17:17.