5 How then was He born, I pray you? “When as His mother Mary was espoused:” He says not “virgin,” but merely “mother;” so that his account is easy to be received. And so having beforehand prepared the hearer to look for some ordinary piece of information, and by this laying hold of him, after all he amazes him by adding the marvellous fact, saying, “Before they came together, she was found with child of the Holy Ghost.” He says not, “before she was brought to the bridegroom's house;” for indeed she was therein. It being the way of the ancients for the most part to keep their espoused wives in their house: in those parts, at least, where one may see the same practised even now. Thus also Lot's sons-in-law were in his house with him. Mary then herself likewise was in the house with Joseph.
And wherefore did she not conceive before her espousal? It was, as I said at first, that what had been done might be concealed awhile, and that the Virgin might escape every evil suspicion. For when he, who had most right of all to feel jealousy, so far from making her a show, or degrading her, is found even receiving and cherishing her after her conception; it was quite clear that, unless he had fully persuaded himself that what was done was of the operation of the Holy Spirit, he would not have kept her with him, and ministered to her in all other things. And most properly has he said, that “she was 'found' with child,” the sort of expression that is wont to be used with respect to things strange, and such as happen beyond all expectation, and are unlooked for.
Proceed therefore no further, neither require anything more than what has been said; neither say thou, “But how was it that the Spirit wrought this of a virgin?” For if, when nature is at work, it is impossible to explain the manner of the formation; how, when the Spirit is working miracles, shall we be able to express these? And lest you should weary the evangelist, or disturb him by continually asking these things, he has said who it was that wrought the miracle, and so withdrawn himself. “For I know,” says he, “nothing more, but that what was done was the work of the Holy Ghost.”
Source: Homilies on the Gospel of St. Matthew (New Advent)