Low levels of PSGs in the maternal circulation have been associated with spontaneous abortion, preeclampsia, intrauterine growth retardation, small for gestational age fetuses, and preterm delivery.231–233 PSG1 is the most widely studied member of PSGs, and has been determined to induce TGF-β activation.234,235 TGF-β inhibits cytotoxicity of CD8+T and natural killer cells and induces differentiation of CD4+Foxp3+Treg cells. This evidence concerns the gene TGFB1 and abortion.