In the Summa Theologiae, St. Thomas Aquinas explains that Christ assumed the deficiencies of human nature; he gave three reasons for this: so as to be able to offer satisfaction for our sins, in order to show the truth of His human nature, and finally in order to provide the example of the virtues (III, q.14, a.1).
But of what deficiencies is he speaking? St. Thomas continues in explaining that Christ’s mission, in particular that of satisfaction, supposes the perfection of knowledge and of grace, which excludes the ignorance and sin that were the lot of the poor sons of Eve.
As for the corporal deficiencies that are not incompatible with this particular perfection of Our Savior, a distinction must be made:
--the common deficiencies, linked to human nature and the common condition of humanity after original sin: hunger, thirst, death, etc. They were assumed by Christ.
--the deficiencies arising from particular causes (like personal sin, a bad anatomical composition, or a particular external cause): they were not assumed by Christ, who never committed a personal sin, in whom bodily perfection was integrated, and who exercised perfect prudence in the conduct of His life.
What about the Mother of Christ?
The principle is simple to state: the Mother of God was subject to the defaults that were not incompatible with her auxiliary mission to the Savior.
Thus, when considering bodily defects:
--For common deficiencies: they are the same in Our Lady as those that Christ assumed—hunger, thirst, fatigue, suffering, death. On the other hand, since Jesus was spared the corruption of the tomb, it would be the same for Mary.
--As for particular deficiencies: the Virgin did not commit personal sin; she possessed a bodily perfection similar to that of Jesus who proceeds from it; finally she possessed a very perfect prudence. Also, theologians deny sickness in Mary, as with Christ.
The Mother of God had no deficiencies linked to sin
The conception of the Virgin was surrounded by a very special providence, which preserved her from original sin, and which gave her a body protected from all deficiencies linked to sin.
--With the goal of harmonizing her body perfectly with her soul;
--But also so as to allow her to accomplish perfectly her mission in all its aspects:
Firstly, so as to give a perfect body to the Son of God. There is a profound analogy between the generation of the Son by the Father and the generation of the body of Christ by the Virgin. Even as the Father produced from His substance a Son who is perfectly the same, likewise the Virgin produced from her substance, through the operation of the Holy Ghost, a Son who is perfectly the same.
Secondly, so that her marvelous union with the Son of God in charity would be founded on an exceptional union in human nature.
Finally, so that this union would produce the most sublime fruits through compassion, the soul of the Coredemptrix is so wonderfully united to her Son that it can be said that they have one and the same heart as well as the same sentiments. Their natural resemblance furnishes the basis of the perfect supernatural union for the accomplishment of the salvation of humankind.
This extraordinary similitude is absolutely unique and without any comparison in human history.