Pregnancy-specific beta-1 glycoprotein 1 (PSG-1) is also known as SP1, PSbG1, or B1G1, and is designated CD66f. PSGs are part of the carcinoembryonic antigen (CEA) family and serve as early biochemical markers of syncytiotrophoblast formation. PSGs, which are produced by syncytial cells in the placenta, are detectable in the maternal plasma as early as 2-3 weeks following fertilization and increase as pregnancy progresses. They are alleged to promote immune tolerance of the semi-allotypic fetus during pregnancy. PSG-1 fosters an anti-inflammatory uterine environment by inducing IL-10, IL-6, and TGF-beta 1 secretion. Additionally, PSG-1 is believed to mediate placental vascular morphogenesis by enhancing VEGF-A production and endothelial tube formation.