ותצחק שרה, "Sarah laughed;⁠" when the month of Tishrey began, six months after the prediction of the angels, and she had not experienced signs of pregnancy, she abandoned all hope of that prophecy becoming true, as there were only six months left for it to become true. Contrary to her husband Avraham, she had lost faith in the prediction. The reason that Avraham maintained faith in the prediction was that he had heard it from God directly, (the angel had spoken to him in the name of the Lord), whereas Sarah, had only been an eavesdropper, and possibly she had not heard correctly. She had assumed that the angels speaking to Avraham had been ordinary human beings. She took a look at her belly and asked mockingly whether such a belly could possibly produce a fetus. As soon as she had done so, in Tishrey, God asked Avraham why Sarah had reacted in such a manner, adding that at the time appointed, i.e. when a year had passed at the time of Passover, in the following year, the angel would return and she would have become a mother. In practice this means that the first time she heard the prediction she had heard it from the mouth of an angel whom she had believed to be a mortal human being. The second time Avraham heard it from God directly, i.e. 'ויאמר ה, "the Lord said;⁠" this was followed by God adding: "Is anything impossible for the Lord to do?⁠"Support for this interpretation can be found seeing that when the angels on the same evening met Lot, Lot offered them unleavened bread, matzot for supper, a hint that the date was the date that would later on become the night of the Seder of the Jewish people. [This is not far fetched at all, as the night when Avraham had fought and secured Lot's freedom, had been the corresponding date, and Lot therefore had good reason to observe it as an anniversary of great significance for him also. Compare the numerous commentaries on Genesis 14,15, ויחלק עליהם לילה, "He divided the night for them.⁠" Ed.] In the Talmud Rosh Hashanah, 11 both Rabbi Eliezer and Rabbi Yoshua are quoted as in agreement that three previously barren women, i.e. Sarah, Rachel and Chanah, all became pregnant on Rosh Hashanah.

בקרבה, "in her heart and mind;⁠"

לאמור, what is the meaning of this word, which literally means: "to say;⁠" [apparently contradicting the previous word, Ed.] According to the plain meaning we should assume that this word here is no different from elsewhere where it appears and means that certain thoughts were to be voiced by mouth. Sarah then would have prepared to voice her opinion of the prophecy which had not come true. In other words: she was ready to voice her ridicule publicly. This would account for the fact that the Torah took her to task for her disbelief more so than it had taken to task Avraham in Genesis 17,17, where he is reported to have reacted similarly to the promise by God that Sarah would become the mother of a son of his. However, he had not intended to go public with his doubts.

אחרי בלותי, "after I have ceased menstruating?⁠" Sarah is almost incredulous that nature would reverse itself in her case. She adds that even assuming that she has experienced this kind of rejuvenation, she knows from her personal experience that her husband has not experienced such symptoms. This latter thought of Sarah is the one the Torah did not reveal to Avraham, not wishing to cause friction between Sarah and her husband by revealing that she considered him impotent. If you wonder about the Torah's choice of the expression היתה לי, "I have experienced,⁠" in the past tense, instead of the expression תהיה לי, in the future tense, this is something we find quite often in the Holy Scriptures.