Why doesn't a pregnant woman reject her fetus?

Recent evidence suggests that the mother's immune system doesn't "realize" that the fetus is foreign even though it gets half its genes from the father.
Answers:
Rejection of offspring would be remarkably disadvantageous from an evolutionary standpoint. Such a trait would be remarkably unlikely to thrive. This is probably freaking obvious, though. I couldn't tell you the biochemical mechanism, which is what I assume you're looking for.
At lowest 15 years ago I read an article which said that,generally speaking,frequent exposure to the father's sperm ( read:genes ) informs the mother's immune system that those 'foreign' genes are not harmful.
So what about conception from a one-night-stand?Well,fortunately,the mother's immune system recognises her own genes within the mix.Our reproductive predisposition is to breed,and so rejecting every foetus would be counterintuitive to the survival of the species.We have evolved to not reject most foetuses.
But a woman may reject many in her lifetime in need knowing ( you may have conceived but it was rejected in your subsequent normal menstrual period ).
It does happen to some women, I can't remember the signature for it, but it's when the baby has a blood type that isn't compatible with the mothers and it make one/both of them sick. You're right though, that doesn't normally happen even if they do have different blood types.
surrounded by a sense there are ways the body does reject it without ridding itself of it- such as pre-eclampsia- there really isn't a true explanation for why some population get it. There's also a rash that a small percentage of women get when have a boy. It's a reaction to the testosterone that the male baby have and the woman's body fights it, thus, breaking out into a terrible rash. I suppose the different blood types and Rh factor, too, could be an example.These may not be exactly the answers ur looking for, but i think they are ways the immune system or systemically are affected.
Published research has identified the molecule phosphocholine which is secreted by the placenta as playing an meaningful role in suppression of the mother's immune system response, thereby not rejecting the placenta. This molecule is added to the surface layer of several proteins, masking them from the mother's immune system
This molecule is also used by parasitic nematodes to suppress the immune system of their hosts. Source(s): http://news.bbc.co.uk/2/hi/health/708129…
Because it is a separate entity to an extent. Its merely recieveing nutrients from its mother.

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