In the last years, a polymorphic family of placenta-expressed proteins has been discovered in ruminant species and used for pregnancy diagnosis. Pregnancy diagnosis is an important part in reproduction management of ruminants. The pregnancy-associated glycoproteins (PAG) are synthesized in the mono- and binucleate cells of the ruminant's trophectoderm. Part of them is released into maternal blood circulation where they can be assayed by different RIA and ELISA systems. Due to large variety of expressed molecules and to large variations in the post-translational processing of the glycoproteins, different immuno-systems present different ability to quantify the PAG released in blood. Recent investigations showed that surprisingly the level of milk production in ruminants can modify the concentration of PAG circulating in blood. On the whole, the data show that the RIA methods are very precise for measuring PAG concentrations in the maternal blood and milk of the ruminants. Different studies clearly indicate that milk can be used for pregnancy diagnosis in small ruminants. The sensitivity and specificity of this method are very high. The results showed the possibility of the use PAG in milk and in blood as pregnancy test. It is especially helpful in the diagnosis of gestation and in detection of embryonic mortality as a non stressed method in the pregnancy management in the ruminants.