Charles Williams, from The Descent of the Dove:
"At the beginning of life in the natural order is an act of substitution and co-inherence. A man can have no child unless his seed is received and carried by a woman; a woman can have no child unless she receive and carries the seed of a man – literally bearing the burden. It is not only a mutual act; it is a mutual act of substitution. The child itself for nine months co-inheres in its mother; there is no human creature that has not sprung from such a period of an interior growth."
And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, and he will reign over the house of Jacob forever, and of his kingdom there will be no end." And Mary said to the angel, "How will this be, since I am a virgin?" And the angel answered her, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy- the Son of God. And behold, your relative Elizabeth in her old age has also conceived a son, and this is the sixth month with her who was called barren. For nothing will be impossible with God." And Mary said, "Behold, I am the servant of the Lord; let it be to me according to your word." And the angel departed from her. 
Had he willed, he could presumably have raised for his Incarnation a body in some other way than he chose. But he preferred to shape himself within the womb, to become hereditary, to own to humanity the flesh that he divinitized by the same principle – “not by conversion of the Godhead into flesh, but by taking of the manhood into God.” By an act of substitution he reconciled the natural world of the kingdom of heaven, sensuality with substance. He restored substitution and co-inherence everywhere; up and down the ladder of that great substitution all our lesser substitutions run; within that sublime do-inherence all our lesser co-inherences inhere.
It is the manner of childbirth. It is the image everywhere of supernatural charity.” It is also the bedrock of intercessory prayer when in a very real way, like Moses of old, we stand in the breach for others, “Therefore he said he would destroy them- had not Moses, his chosen one, stood in the breach before him, to turn away his wrath from destroying them” thus fulfilling the divine command, “Bear one another’s burdens.”