Genesis 18:12 - Joseph Benson’s Commentary on the Old and New Testaments

Sarah laughed within herself It was not a laughter of faith, like Abraham's, (Genesis 17:17,) but a laughter of doubting and distrust. The great objection which Sarah could not get over was her age. I am waxed old And past child-bearing in a course of nature, especially having been hitherto barren, and, which magnifies the difficulty, My lord is old also. Observe here, that Sarah calls Abraham her lord, and the Holy Ghost takes notice of it to her honour, and recommends it to the imitation of all Christian wives, 1 Peter 3:6, Sarah obeyed Abraham, calling him lord, in token of respect and subjection.