Pregnancy-associated glycoproteins (PAGs) are powerful pregnancy markers in domestic cattle. These proteins are expressed in mono- and binucleate trophoblast cells from the first days of gestation until calving. Different molecules were identified as being expressed at various stages of pregnancy. However, up to date, their functions and activities during pregnancy were not yet established. Specific RIA tests were developed (classic and alternative RIA) and used to measure the concentration of these glycoproteins in blood during gestation and the postpartum period in cattle. In maternal blood, PAGs rise to detectable levels from days 24 to 28 after fertilization. A recent study indicated that PAGs can also be detected in milk samples. However, concentrations in milk are much lower when compared to those of plasma.