5 And when He says, “Hear, O house of David,” He performed the part of one indicating that He whom God promised David that He would raise up from the fruit of his belly (ventris) an eternal King, is the same who was born of the Virgin, herself of the lineage of David. For on this account also, He promised that the King should be “of the fruit of his belly,” which was the appropriate [term to use with respect] to a virgin conceiving, and not “of the fruit of his loins,” nor “of the fruit of his reins,” which expression is appropriate to a generating man, and a woman conceiving by a man.
In this promise, therefore, the Scripture excluded all virile influence; yet it certainly is not mentioned that He who was born was not from the will of man. But it has fixed and established “the fruit of the belly,” that it might declare the generation of Him who should be [born] from the Virgin, as Elisabeth testified when filled with the Holy Ghost, saying to Mary, “Blessed are you among women, and blessed is the fruit of your belly;” the Holy Ghost pointing out to those willing to hear, that the promise which God had made, of raising up a King from the fruit of [David's] belly, was fulfilled in the birth from the Virgin, that is, from Mary.
Let those, therefore, who alter the passage of Isaiah thus, “Behold, a young woman shall conceive,” and who will have Him to be Joseph's son, also alter the form of the promise which was given to David, when God promised him to raise up, from the fruit of his belly, the horn of Christ the King. But they did not understand, otherwise they would have presumed to alter even this passage also.
Source: Adversus haereses (New Advent)