Microchimerism and the Assumption of the Blessed Virgin Mary. This book began as a question that will not leave a Catholic imagination once it has been asked. Science now tells us that a child does not depart the mother’s body as completely as the cutting of the cord would suggest. Some of the child remains. Some of the mother remains in the child. The bond is not only legal, emotional, or sacramental. It is cellular. If that is true of ordinary motherhood, it is true, by the logic of the Incarnation, of the most extraordinary motherhood the world has ever known. The Word became flesh in the womb of the Virgin Mary. He had a real human body, a real placenta, a real span of months beneath her heart. What every child leaves in his mother, He would have left in her. What every mother leaves in her child, she would have left in Him. From that single inference a larger thought follows, and it is the thought these pages exist to unfold. When Christ ascended into heaven, His glorified body took with it every cell that belonged to Him, including whatever cells of His Mother still lived in His flesh. What His Ascension did not take was the colony of His own cells still living in her. Those cells carried His human genome. They were, in the strict biological sense, remnants of the Holy One still on earth. The dogma of the Assumption then appears not as an ornamental privilege, but as the last act of a filial love that would not leave one cell of His body behind. I must say at once what this book is not. It is not a new dogma. It is not the Church’s official reason for the Assumption. Pope Pius XII defined that mystery in 1950 from Scripture, Tradition, and the long instinct of the faithful, without a microscope in his hand and without any need of one. The Church has not formally addressed microchimerism in relation to Our Lady.7, 15 What follows is an argument of fittingness: a way of seeing, with the help of a young science, something the Church already knew by a much older light. A claim that touches both the laboratory and the altar owes both a careful hearing. These pages therefore walk slowly through the history of the discovery, the tissues in which fetal cells have been found, the honest quarrel about what those cells are doing, the councils that insisted on Christ’s true flesh, the papal constitution that defined the Assumption, and the limits that keep a meditation from becoming a novelty.