And when [a pregnant woman's immune system] does attack the fetus (probably the underlying cause of most complications during pregnancy involving an otherwise viable embryo or fetus), it doesn't do so to prevent the mother from having a pregnancy that's going to mess up her CV, but because the system of checks and balances failed.
I'm not actually making that up, not entirely at least. I remembered after writing the above post that, many moons ago, I'd run into a paper or such that argued pretty much just that, and I managed to dig it up (minus "prevent the mother from having a pregnancy that's going to mess up her CV", add "enable females to unwittingly terminate pregnancies that were not in their long-term reproductive interests", potayto potahto). Turns out it was a book chapter, and fulltext is available via
researchgate.
I considered it then, as I do now, a prime example of hyperadaptationism, a prime example of a certain brand of evolutionary psychology that's so bad it's giving both "evolution" and "psychology" a bad rep, and indeed a prime example of bad science. I can only hope it was published on April 1, but have found no indication that it was. Cambridge University Press isn't what it used to be either.
The book chapter is titled "Preeclampsia and other pregnancy complications as an adaptive response to unfamiliar semen", and it only goes downhill from there. There are probably people in this thread better qualified in immunology and/or procreative medicine than I am, but very coarsely, preeclampsia is a condition that arises when the placenta (which is made up of embryonic tissue, ie tissue that is alien to the mother) "invades" the uterus and receives a stronger-than-usual immune response. It's a fairly common complication during pregnancy in humans, rare in other apes, and unknown in other mammals. Common wisdom has it that the "invasion", that is the formation of a higher surface area through appendices extending into the uterine tissue, becomes necessary in humans specifically to achieve a higher nutrient flow needed to develop our babies' oversized brains. It has been observed for quite some time that preeclampsia is more common in first pregnancies, and more recently that that's a simplification, as it also common in subsequent pregnancies of the mother in those cases where the father is a different one. Then there's some stuff about barrier contraception and sperm donors etc., and the statistical data appears to be fairly solid: Prior exposure to the father's semen significantly reduces the risk of preeclampsia. Which is perfectly explained by the immune system doing what it's best at: attacking foreign antigens, but with room for desensitisation reducing the response.
Instead, they come up with the speculative idea that preeclampsia is an adaptive response targeted at minimising the chance of bringing to term a pregnancy where the father is unlikely to contribute to child-rearing. Occam would turn in his grave, never mind that preeclampsia causes about one maternal death for every seven fetal deaths and is, in developing countries, responsible for 1/5 to 1/4 of all maternal deaths.
Sure, if you have reason to believe that the mechanistic explanation is insufficient, go ahead and propose an additional factor. If you did the math and found that the maximal plausible effect size of the expected immune response to fetal tissue
literally, physically, invading the uterus is insufficient to explain the immune response, and/or that everything we know about immune system desensitisation is insufficient to explain the "familiar sperm" effect, it's your turn. You'd still need to demonstrate that it's worth it, given the high cost in maternal mortality, but it's a start. However, they
aren't even trying to show any of that. The paper doesn't contain a single formula. Sure, there are many aspects in life, and many strands of scientific research, that don't require formulae. Postulating an evo-psych explanation for something that is, on the face of it, sufficiently explained by immunology, however, isn't one of those.
If anything, I find the exact opposite scenario much more plausible: Our mating patterns and hidden ovulation as an preeclampsia avoidance strategy, rather than preeclampsia as a strategy to avoid (births resulting from) matings that don't fit our pattern. Starting from the high nutrient demands of our oversized fetal brains which necessitate a more "invasive" implantation of the placenta into the uterus, and taking into account vertebrate-universal properties of our immune system such as how it generally reacts to foreign tissue and how it can be desensitised by constant low level exposure to antigens, the peculiarities of our mating system and the abnormally low fecundity of our females might very well be "preeclampsia-avoidance strategies", rather than preeclampsia being "one-night-stand pregnancy avoidance strategies". If you look compare
homo sapiens with most other extant mammals, two things are very evident (and others, but those are the ones relevant here): Our females don't show when they are receptive, and more often than not, they
don't get pregnant even after having unprotected sex at just the right time. Ask any cattle breeder: he'll now when his cows are in heat, and if he matches one with a bull while in heat and she doesn't get pregnant, he'll likely consider her to suffer from some disorder. Human women, in contrary, don't show when they are are ovulating, and more often than not
don't get pregnant even after having unprotected sex while they are. In that particular respect, even female chimps are very much like cows and unlike human females. And that human state of affairs doesn't come for free: it's basically what causes regular menstruations. If you are a man and think that's a fair price: ask any woman. It does, however, make it so that most women's immune system will be fairly familiarised with the father's sperm by the time they do get pregnant, assuming most of her sex is repeatedly with the same small set of men. Here's a paper that discusses just that:
https://pubmed.ncbi.nlm.nih.gov/11900595/
There are some things evolution can easily change without deleterious effects. The fact that our immune system is hostile to foreign tissue just isn't one of them.