Cancer immunotherapy relies upon the immune system recognizing and killing cancer cells. Tumors can elude recognition by readapting existing mechanisms of immune control and suppression. Here we explore the hypothesis that cancers repurpose the immune suppression employed during pregnancy to protect the allogeneic fetus. Those mechanisms are reviewed and shown to be employed both in pregnancy and by tumors. Pregnancy specific glycoproteins (PSGs) produced by fetal trophoblasts are also synthesized by a large number of tumors, which are associated with a poor overall survival of the patient. The family of PSGs may well be a useful target for future checkpoint therapy.

Pregnancy-specific glycoproteins; T-cell tolerization; Checkpoint therapy