Genesis 18:11

חדל להיות means [that] it had ceased from her.

אורח כנשים means the way of menstruation.

But Abraham and Sarah were old, they had mounted (alu) in days, and with Sarah the way of women had ceased.

‘BA’IM BAYAMIM’ (ADVANCED IN DAYS). In his youthful days a man is called “standing in days,” and they are referred to as “his days” because they belong to him, just as in verse The number of thy days will I fulfill. But when he gets old and has lived longer than most people of his generation, it is said of him that he is ba bayamim, [literally, “came into days”], because it is as if he came into another land, travelling from and arriving in a city each and every day.

IT HAD CEASED TO BE WITH SARAH AFTER THE MANNER OF WOMEN. This is the time of pregnancy, for after menstruation has ceased due to old age, a woman will not become pregnant.

ואברהם, this line has been written only in order to account for the reason of Sarah’s laughter.

באים בימים, this is the standard mode in which the Torah describes people who feel that aging has affected their general physique and imposed limitations on them. What the Torah means is that both Avraham and Sarah had reached the years when other people are subject to the limitations of old age. These are the years when the soul begins to separate from the body.

אורח כנשים, the signal of old age for women, i.e. cessation of their menstrual cycle. The Torah uses a euphemism to describe that phenomenon. Rachel used the same euphemism when she explained to her father Lavan why she could not let him search her camel (Genesis 31,35). During old age, due to the body drying out, the flow of blood stops, i.e. a woman no longer ovulates. Our sages in Ketuvot 10 relate that the volume of such a flow of blood is in direct relation to the number of children a woman is likely to bear. There was a well known family by the name of Dorkaty in those days, whose female members never experienced either menstrual blood, nor the blood associated with the puncturing of the hymen. This is why the family was named Dorkaty, meaning that something was radically wrong. None of the females in that family ever had children, the reason being attributed to this abnormal condition of their not menstruating.

חדל להיות לשרה אורח כנשים, “Sarah had ceased to have the periods of women.” She suddenly saw menstrual blood again, something that does not occur in women over a certain age which she had long passed attaining. The meaning cannot be that “she did not see such blood,” as in his commentary on verse eight, Rashi told us that she menstruated, and that this was the reason why she did not bake and offer bread as she had been commanded to do by her husband (Compare verse 6)