Does a mother's blood pass through the placenta to her unborn baby?
The major components of a mother's blood never pass through the placenta to the fetus. Only small chromosomes and genes carrying inherited characteristics go through. This explains why none of Mary's blood coursed through baby Jesus; His blood system came from His Father, just as any child's blood system does.
Les explains this by comparing the placenta to fencing a garden: netting keeps out rabbits and dogs, but insects and small mice still get through. Likewise, the major components of the mother's blood never go through the placenta to the fetus, but some small chromosomes and genes that carry inherited characteristics do pass through. He applies this to Mary as the physical fleshly mother of the Lord Jesus: none of Mary's blood, which was just as corrupt as any other woman's because she was a sinner, coursed through the baby Jesus. Instead, His blood system came from His Father, just like any other child's blood system comes from the father. Les notes this boggles many people's minds and ties it back to the promise of the seed of the woman in Genesis 3:15, then moves to Matthew 1:1.