system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Is a pregnant mother's blood kept separate from her fetus' blood?

I have searched online multiple times to see whether a pregnant mother's blood and her fetus' blood are kept separate. I've even looked in textbooks, and often it's implied but never said outright and plainly in those terms.

I've looked online, but I've not yet found a reputable source that spells it out clearly.

Are a pregnant mother's blood and her fetus' blood kept separate? Why or why not?
gpt
There is a placental blood barrier, yes.

> 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. If the mother and the fetus had different blood types, they might both die if their blood mixed.

Wikipedia.