The paradox of maternal tolerance to the semi-allogeneic fetus is such
an old question that there are many papers out there. You could start at
the beginning with Peter Medawar's work in the mouse ca. early 1950's.
More recent papers include a Nature paper in 1976 by Faulk et al
regarding suppression of MHC molecule expression on
syncytiotrophoblasts, and numerous recent papers on HLA-G, which many
think plays a role in maternal tolerance.
Hope this helps.
jon