24 In what remains we have the appointment of the Father's will. The Virgin, the birth, the Body, then the Cross, the death, the visit to the lower world; these things are our salvation. For the sake of mankind the Son of God was born of the Virgin and of the Holy Ghost. In this process He ministered to Himself; by His own power— the power of God— which overshadowed her He sowed the beginning of His Body, and entered on the first stage of His life in the flesh. He did it that by His Incarnation He might take to Himself from the Virgin the fleshly nature, and that through this commingling there might come into being a hallowed Body of all humanity; that so through that Body which He was pleased to assume all mankind might be hid in Him, and He in return, through His unseen existence, be reproduced in all. Thus the invisible Image of God scorned not the shame which marks the beginnings of human life. He passed through every stage; through conception, birth, wailing, cradle and each successive humiliation.
Source: On the Trinity (New Advent)