26 But lest perchance fastidious minds be exercised by cradle and wailing, birth and conception, we must render to God the glory which each of these contains, that we may approach His self-abasement with souls duly filled with His claim to reign, and not forget His majesty in His condescension. Let us note, therefore, who were attendant on His conception. An Angel speaks to Zacharias; fertility is given to the barren; the priest comes forth dumb from the place of incense; John bursts forth into speech while yet confined within his mother's womb; an Angel blesses Mary and promises that she, a virgin, shall be the mother of the Son of God.
Conscious of her virginity, she is distressed at this hard thing; the Angel explains to her the mighty working of God, saying, The Holy Ghost shall come from above into you, and the power of the Most High shall overshadow you. The Holy Ghost, descending from above, hallowed the Virgin's womb, and breathing therein (for The Spirit blows where it lists), mingled Himself with the fleshly nature of man, and annexed by force and might that foreign domain. And, lest through weakness of the human structure failure should ensue, the power of the Most High overshadowed the Virgin, strengthening her feebleness in semblance of a cloud cast round her, that the shadow, which was the might of God, might fortify her bodily frame to receive the procreative power of the Spirit.
Such is the glory of the conception.
Source: On the Trinity (New Advent)