There’s a Wikipedia article on “Immune tolerance in pregnancy” with interesting info, though it’s often hard to view external references because of paywalls that show only abstracts or less. Anyhow they talk about the view that the fetus and placenta ”...may be viewed as unusually successful allografts, since they genetically differ from the mother.” The main article on “Pregnancy” at Wiki makes reference to a study of graft-versus-host reactions in pregnant mice.
Unfortunately Google searches for scholarly articles on the topic bring up many results behind high ($33 for 1-day access to an article?) paywalls. If you’re lucky you get an abstract. Here’s a freebie from 1982:“These results support the concept of pregnancy-associated suppression of cell-mediated immunity.”
I found an out-of-date medical textbook from 1999 titled Chestnut’s Obstetric Anesthesia (definitely not the Kindle edition). In a 25-page chapter on “physiologic changes of pregnancy” there are only a couple of paragraphs about the immune system. Of interest to our discussion
Polymorphonuclear leukocyte function is impaired during pregnancy, as shown by depressed neutrophil chemotaxis and adherence.113 This may account for the increased incidence of infection during pregnancy and the decreased incidence of symptoms in some pregnant women with autoimmune diseases (e.g., rheumatoid arthritis). However, pregnancy does not appear to be associated with suppression of auto-antibody production.114 Although the serum concentrations of immuno-globulins A, G, and M are unchanged during gestation, humoral antibody titers to certain viruses (e.g., herpes simplex, measles, influenza A) are decreased.115
No doubt much has been learned about this since 1999.