So to answer this question, let's begin with the following diagram. So this is our fetus. This is the placenta that connects the bloodstream of the fetus to the bloodstream of our female, of the mother. And this is the blood vessel of our mother. Now, normally, because the red blood cells are too large, they cannot pass across the placental membrane. But during pregnancy, during childbirth, that is, there is a small likelihood that there can be exchange of red blood cells.