21 And Isaac intreated the LORD for his wife, because she was barren: and the LORD was intreated of him, and Rebekah his wife conceived.
[AD 253] Origen of Alexandria on Genesis 25:21
First of all consider why it is that many holy women in the Scriptures are related to have been barren, as Sarah herself, and now Rebekah. But also Rachel, Israel’s beloved, was barren. Hanna also, the mother of Samuel, is recorded to have been barren. But also in the Gospels Elizabeth is related to have been barren. Yet in all these instances this term is used because after sterility they all gave birth to a holy person.

[AD 345] Aphrahat the Persian Sage on Genesis 25:21
Isaac too demonstrated the power of prayer when he prayed over Rebekah, and she gave birth.

[AD 397] Ambrose of Milan on Genesis 25:21
Now Rebekah conceived and by her patience untied the knot of sterility. Let us consider what her prophetic and apostolic soul brought to birth, and how. “She went to consult the Lord,” because the children leapt up in her womb, and she received the reply, “Two nations are in your womb.” For of herself she presumes nothing but invokes God as supreme protector of her counsels; filled with peace and piety, she joins two nations together by her faith and by prophecy and encloses them in her womb, so to speak. Not without reason is she called sister rather than wife, because her gentle and peaceable soul enjoys a reputation for affection common to all rather than for union with one individual and because she thought that she was bound to all rather than to one.

[AD 407] John Chrysostom on Genesis 25:21
One question is worth raising initially: If she and her husband were conspicuous for their good life and both concerned for chaste living, why was she barren? We cannot find fault with their life or say barrenness was the result of sin. To grasp the full extent of this remarkable circumstance, remember that it was not only herself who was barren but also the good man’s mother, Sarah; and not only his mother but also his daughter-in-law—I mean Jacob’s wife Rachel.What is the meaning of this gallery of sterile people? All were good people, all virtuous, all given testimony by God; of them he said, “I am the God of Abraham, the God of Isaac and the God of Jacob.” And blessed Paul says, “Hence God is not ashamed to be called their God.” There is great commendation of them in the New Testament; great praise of them in the Old. In each case they were distinguished and remarkable men, yet all had barren wives. They spent a long period in a childless condition. So whenever you see a man and wife of virtuous life experiencing childlessness, whenever you see pious people devoted to religion yet childless, don’t think it is the result of sin. After all, many reasons for God’s designs are beyond our understanding, and we ought to thank God for everything and brand as wicked only those living in sin, not those without children. It frequently happens, in fact, that God works things for our good without our realizing the reason for what happens. Hence in every case we should marvel at his wisdom and praise his ineffable love.

[AD 407] John Chrysostom on Genesis 25:21
These things are told for our benefit so that we may give evidence of much goodwill and not pry into God’s plans. Yet we need to explain the reason why these women were barren. What, then, is the reason? So that when you see the Virgin giving birth to our common Lord you may not be incredulous. Exercise your mind, it is saying, on the womb of these sterile women, so that when you see an infertile and sealed womb opened for childbearing by God’s grace, you may not be surprised to hear that a maiden gave birth. Or rather, feel surprise and amazement but don’t refuse faith in the marvel. So when the Jew says, “How did the Virgin give birth?” say to him, “How did the sterile old woman give birth?” In that case, you see, there were two impediments, her advanced age and the imperfect condition of nature, whereas with the Virgin there was one impediment, her not having experienced marriage. Consequently the barren woman prepares the way for the virgin.

[AD 420] Jerome on Genesis 25:21
(Verse 21.) And Rebecca his wife conceived, and the sons within her were moved. Concerning this movement, the Seventy Interpreters rendered it as ἐσκίρτων, which means they were playing or kicking; which Aquila translated as they were breaking in the womb. But Symmachus rendered it as διέπλεον, which means they were carried on the surface like a ship.

[AD 735] Bede on Genesis 25:21
Thus Isaac, the son of the promise, who as a figure of our Redeemer became obedient to his father even unto death, was born of parents who were old. He had a mother who had long been barren. Thus Jacob, the patriarch Joseph, Samson, [who was] the bravest of the chieftains, and Samuel, [who was] the most distinguished of the prophets, [all] had as their progenitors [mothers who were] for a long time barren in body but always fruitful in virtues. In this way their dignity would be known from the miraculous nativity of those who were born, and it might be proven that they would be famous in their lives, since at the very outset of their lives they transcended the norms of the human condition.