Good question vary good question but I think the blood never mix because One of the placenta's jobs is to make sure blood from the mother and fetus never mixes. The placenta acts as an exchange surface between the mother and the fetus. Nutrients and oxygen are passed over by diffusion only. If the mother's and fetus's blood mixed, it could be deadly for both of them. I hope I helped you and am sorry if it’s too late I just ate to much sugar it is like 11 at the night but still I helped you and that’s good. Good luck
