A couple people have asked me recently how the baby's blood type is determined. If the baby depends on Stephanie for food, oxygen and disposal of waste and these pass through the umbilical cord, does blood also move back and forth, and if so, does this determine the baby's blood type? The answer to both questions is no. Although the baby's blood and Stephanie's blood come very close, they never intermingle. They are close enough for the food, oxygen and waste to pass between them but without their blood coming in contact. This is the role of the placental barrier.
As for the baby's blood type, it is determine purely from genetics. Since Elizabeth is AB and I am A, then the baby can only be A, B or AB.
That's your biology lesson for Tuesday!
More on the placental barrier
More on fetal blood flow: