If I could just add a little context to the discussion.
there is a quote from St John of Damascus being discussed "not developing the fashion of the body by gradual additions but perfecting it at once."
It would help the discussion I think to place this in the philosophical and medical context of the time. At this time Aristotle's view of man's development was widely accepted in some circles. Aristotle in examining embyo's noticed that in successive embyo's more parts were added. He came up with a theory of empigensis in which as the embyo developed it went through a process of gradual ensoulment - first a vegetative, then and animal and finally once the body was fully developed a rational soul. His belief was that a male baby received a soul after 40 days and a female after 90 days. There was a competing theory by other philosophers called preformation where everything needed for development was there from the start, but in rudimentary form.
What St John is specifically speaking against is Aristotle's idea of successive life principles being added - He is asserting that Christ was perfect - ie. complete- at conception.
It seems that Thomas Aquinas bought into Aristotle's views of a human embryo receiving a soul only at 40 days after conception. see here
However, St Maximos the Confessor along with St Gregory of Nyssa and other fathers denounce this view of our development. (St Basil in the one letter where this is mentioned merely admonishes those arguing not to worry about formed or unformed) teaching very firmly that soul and body come into being together and this is the theory that is pretty much universally accepted today. We do not accept - either medically or philosophically or theologically that a human embryo is something less than human at conception, but that it is indeed fully human.
Where that leaves us in terms of Christ's formation in the womb is still up in the air.
In Ambigua 42 St Maximus states "For He does not follow others to perfection, neither should it be thought that He has arrived at perfection through progress, for He alone by nature is absolutely perfect and the creator of every perfection, and He does not wait to receive perfection together with others, for unlike the totality of beings He has no need of making any progress in anything. ... otherwise.... He is not the pioneer and savior of those who are being saved, since He did not show us in Himself, definatively and mystically, the goal of our perfection, but to the contrary will appear to be simply one among many that are saved..."
This is important because there are those out there (including a few Orthodox theologians) who are saying that Christ needed to be perfected as a man.
St Maximus, though, unlike Thomas Aquinas rejects Aristotle's ideas of development and says "I am extremely apprehensive about admitting this particular argument since the logical implications might justly make me subject to dreadful accusations since it will lead me to utter what is not lawful...
to say that our Lord and God, who truly deigned to become man like us but without sin, became at the moment of his conception a human being without soul or intellect and that He remained in this condition for 40 days. .... and this is why before everything else I espouse the doctrine of coexistence, rejecting each of the
other views that are at variance both with each other and with the middle position to which I adhere, having for my argument's advocate and unerring teacher the very same creator of nature, in the mystery of his embodiment, who truly became man, and who confirmed in Himself that His human nature subsists in its full
perfection simultaneously with its coming into being at the moment of its creation...."
St Maximus here is saying that since Christ became a full human being at conception, therefore we also must receive our soul and be complete human beings at conception. It is a slightly different approach then those who accept that we only come to perfection gradually and therefore see Christ as coming into being in a different way then us.
I think there is still a debate about what "perfection" or "completion" entails in our own development. Probably those who do not see the embryo as complete or perfect at conception are going to assert that Christ's conception was physically different, and those who believe that a man is formed complete or perfect at conception will allow that physically Christ could have gone through the stages of development as an embryo just as he grew in stature after birth. It depends I think on our conception of complete. Both these views can find support in our tradition. Likewise there may be debate between those who when we say that Christ was fully man and talk about the recapitulation - there are different ways this has been viewed - some more materially (and in this case those seeing it this way would want to assert that Christ had to go through the same stages of development as we do as embyros) and some less so. St Maximos himself sees Christ's economy more in terms of the principle or logos of our nature rather than recapitulation in terms of physical development.