John Calvin on Genesis 18:11

11. Were old, and well stricken in age Moses inserts this verse to inform us that what the angel was saying, justly appeared improbable to Sarah. For it is contrary to nature that children should be promised to decrepit old men. A doubt, however, may be entertained on this point, respecting Abraham: because men are sometimes endued with strength to have children, even in extreme old age: and especially in that period, such an occurrence was not uncommon. But Moses here speaks comparatively: for since Abraham, during the vigor of his life, had remained with his wife childless; it was scarcely possible for him, now that his body was half dead, to have children; he had indeed begotten Ishmael in his old age, which was contrary to expectation. But that now, twelve years afterwards, it should be possible to become a father, through his aged wife, “Patrem ex vetula effoetaque muliere fieri posse.” was scarcely credible. Moses however chiefly insists upon the case of Sarah; because the greatest impediment was with her. It ceased, he says, to be with Sarah after the manner of women. The following passage is not translated: — “Quo genere loquendi verecunde menses notat qui mulieribus fluunt. Una autem cum fluxu menstruo desinit concipiendi facultas.” With this expression, he soberly speaks about the monthly stream of the women. At the same moment with this, the possibility of conceiving ceases.