John Calvin Commentary Genesis 18:11

"Now Abraham and Sarah were old, [and] well stricken in age; it had ceased to be with Sarah after the manner of women." — Genesis 18:11 (ASV)

Were old, and well stricken in age. Moses includes this verse to inform us that what the angel was saying rightly seemed improbable to Sarah. For it is contrary to nature that children should be promised to very old men. A doubt, however, may arise on this point regarding Abraham, because men sometimes possess the strength to have children, even in extreme old age; and especially in that era, such an event was not uncommon.

But Moses here speaks comparatively. Since Abraham, during the prime of his life, had remained childless with his wife, it was scarcely possible for him to have children now that his body was as good as dead. He had, indeed, fathered Ishmael in his old age, which was contrary to expectation. But that now, twelve years later, it should be possible for him to become a father through his aged wife was scarcely believable.

Moses, however, chiefly emphasizes Sarah's situation, because the greatest obstacle lay with her. It ceased to be with Sarah after the manner of women, he says. With this expression, he discreetly refers to women's monthly periods. When this ceases, the ability to conceive also ends.