Genesis 18:11

They were both old and advanced in years, and it had ceased to be with Sarah after the manner of women. To highlight the growth of heavenly power, he says that both were old, and moreover that Sarah, having been barren, was now also deprived of her menstrual blood, because of which she could no longer conceive, even if she had not been barren. Further, if a woman is of such an advanced age that the usual flow of women still occurs to her, she can conceive from a young man, but not from an old man; although the old man can still beget from a young woman, as Abraham, after Sarah's death, was able to beget from Keturah, because he found her age to be vigorous; this therefore is what the Apostle clearly commends when he says: "He believed against hope in hope, that he might become the father of many nations, according to what was said to him: Thus shall your offspring be; and not weakening in faith, he did not consider his own body already dead, being about a hundred years old, and the deadness of Sarah's womb; in the promise of God he did not doubt" (Romans 4:18). For this reason he says that Abraham's body was already dead, because he could still beget from any woman who still had some final period of time left for bearing children at that age: we must understand the body as dead for some purposes, not for all; for if for all, then it is not the old age of the living, but the corpse of the dead.

HOW ABRAHAM MIRACULOUSLY HAD A CHILD OF SARAH. — And Abraham and Sarah were old, advanced in days, and the custom of women ceased with Sarah . The age of elderly people is less that those who are old, although the name of old people is also given to people who are simply advanced in age. Now, if it is true, as some doctors assert, that an elderly husband cannot have the children of an elderly woman, even if she is still experiencing what usually happens to women, We can understand Abraham's astonishment at the promise of a son Cf. Gen. 17:17), and the reflection of the Apostle (Cf. Rom 4:19), who sees it as a miracle, since he says that the body of Abraham was already dead. It must not be imagined, however, that the body of Abraham was dead, to such an extent that he could not have had the children of a young woman; But it was so in the sense that it could not have been of a woman advanced in age. If he had children of Céthura, it is because he took still young. Doctors say that a man whose generative vigor has fallen cannot have children with an elderly woman, even if the woman still has menstruation; but you can have them with a young woman. And, on the other hand, an elderly woman, although she still has menstruation, cannot have children with an elderly man, may have in a union with a young man. Therefore, that act of Abraham was a miracle, because, according to what we have said, the body of that man was already dead, and the woman was of an age so advanced that she no longer had menstruation. Now, if anyone would insist on interpreting literally what the Apostle says that "Abraham's body was already dead", and, in saying, dead he understood it in the sense that he no longer had a soul, but was a corpse, he would be sadly wrong. Therefore, the question is solved as we say. On the other hand, we are rightly struck by the fact that the Apostle speaks of a dead body and says that it was a miracle that he had descendants, being as Abraham was a middle-aged man, according to what men lived then, and then, afterwards, he had children with Céthura. [