"As Pius's larger discussion of the debates over sanguis Christi make clear, blood as lineage or collateral relationship is not a very important symbol or image in devotional or academic discourse. A very different philosophical and scientific understanding of blood as descent or continuity is, however, crucial. According to medieval theories of physiology the fetus is formed from maternal uterine blood, animated by the blood or seed of the father and is fed by blood, both in the womb and from the breast. (All bodily fluids are some form of blood in medieval medical theory.)

"Hence, in a startling sense, the blood from which the individual is constituted is [considered] female; the body is the mother's blood. As we have seen, John of Capistrano argued for the high ontological status of sanguis Christi by stressing the formation of Christ's body entirely from the pure menstrual blood of His mother's womb. This sense of Christ's body as formed [...] from Mary's blood had theological ramifications for Christology and Mariology (especially the doctrine of the Immaculate Conception.)

"It also spilled over into devotion. In a striking phrase from a decree of Abbot Berthold of Weingarten concerning the Saturday mass in honor of the Virgin, Christ's body and the stuff of Mary's womb are so completely assimilated that Christ's body almost becomes blood [...]:

Therefore, we are bound to ... [honor her], who, providing to the whole world the price [pretium] of human redemption and pouring forth from her virginal womb for the Christian faithful the universal cure [...] granted us a pledge of hope and a bodily token in His worshipful blood.

"[...] Christ's receiving of us into His heart, which should be matched by our reception of Him into ours, was understood less as re-ceiving, but con-ceiving. Hence, the adult Christ was imaged sometimes [...] as a pregnant male. Not only extravagant mystical visions but also simple prayers were quite graphic in their descriptions both of Christ pregnant with souls and of souls pregnant with Christ. [...] [In regards to Christ's wounded side on the cross,] it also seems to imagine the blood of the side-wound not as the coagulating ooze of a dead body but as birthing blood, living and red."

~Caroline Walker Bynum, Wonderful Blood: Theology and Practice in Late Medieval Germany and Beyond, 2007.