[PatrickHenry]

It is my understanding that, while nutrients and other components pass from the mother's blood to the child's, and waste products pass from the child's blood to the mother's, their actual blood does not intermingle.

Not exactly. If there were an artery directly connecting them, it would rupture at birth and the result would be catastrophic. Instead, the fetus is connected by the umbilical cord to the uterine walls at the placenta, where there is considerable interchange of material, including nutrients, antibodies, oxygen, waste, etc. The mother's blood and the blood of the fetus are in contact, however, and these exhanges occur through capillaries in the placenta. The article in question seems to indicate a total separation of the circulatory system of the fetus, in order to support the author's theological point that: "That means the blood of Mary that would have been marred by sin did not mix with the perfect blood of Christ shed on the cross". That may be good theology, but it is very far from the biological situation. Were there no circulatory connection, the fetus would starve, but long before that it would die of asphyxiation.

[VadeRetro]

Didn't see your reply before I posted mine. I didn't realize blood cells migrated across the barrier, although come to think of it that would seem to be necessary for oxygen transport.

[VadeRetro]

Sorry guy! You had me going, but me right, you wrong.

At the placenta the fetal circulation comes close to the maternal circulation, but there is no actual contact or mixing of blood. Villi formed by the syncytiotrophoblast and cytotrophoblast project towards the maternal decidua basalis. The main shaft of each villus becomes anchored to the decidua basalis, while secondary villi project into the blood space of the placenta. The blood space is to some extent subdivided by placental septa which cause the fetal surface to form units, the cotyledons. The blood in the intervillous space is supplied by spiral arteries which provide a pulsatile flow of maternal blood. The blood flows through the intervillous space to be drained from the placenta by endometrial veins. The villi are supplied with fetal blood by branches of the umbilical arteries.

Placenta and Membranes.

[Ignatz]

That means the blood of Mary that would have been marred by sin did not mix with the perfect blood of Christ shed on the cross

The author may have erred, but not for the reasons you cite. Mary was free from original sin. That is from whence the concept of the "Immaculate Conception" is derived. Immaculate Conception does not, as many believe, perhaps even the author, refer to Mary's virginity, but rather to her being free from original sin. Unfortunately, the rest of us are not so lucky.
Anyway, the mixing of her blood with Jesus' is a moot point.
It would be interesting to speculate, then, that Jesus died on the cross for the salvation of mankind, with the exception of his own mother (who didn't need it)!