Dr. Gerard M. DiLeo wrote an article entitled “Anatomy of a Fetus: The Placenta” at: https://www.babble.com/pregnancy/anatomy-fetus-placenta/

In this article he states:

“The placenta is part of the communication between the fetus and the expectant mother. Most people tend to think of this communication as the route of exchange where the mother’s blood and the fetus’s blood mix and exchange, but this is a myth. The fetal blood and maternal blood do not mix. In fact, if this were to be the case, there would be such immunological protest from the mother that she would soon make enough antibodies to the baby’s blood to destroy the pregnancy. So what exactly is meant by communication between the fetus and mother? And if no blood is exchanged, how do oxygen and nutrition get passed on to the developing baby?”

Later in the article he states:

Another thing that helps is the complete separation of the maternal and fetal circulation. As stated above, there is no direct connection between the fetal bloodstream and the maternal bloodstream. The placenta grows into the maternal uterine lining, but no blood vessels connect up to the mother’s, as was once thought. Instead, the fetal red blood cells end their journey in a U-turn in very vascular capillaries; these capillaries sit in puddles, so to speak, called intervillous spaces. These are puddles of maternal blood that bathe the lining of the placental capillaries. The fetal tissue, thus bathed, takes the developing baby’s needs out of the maternal blood, leaving the rest for the mother.”

As a result, Mary’s human blood did not mix with the pure blood of Jesus.