Equine chorionic gonadotropin (eCG) is a widely used hormone to synchronize the female cycle and induce estrusestruses in livestock breeding. eCG is a heterodimeric glycoprotein composed of non-covalently linked α- and β-chains whose glycosylation profiles determine the in vivo activity of the hormone. The commercially available eCG products are crudely purified from the serum of pregnant mares, hence called pregnant mare serum gonadotropin (PMSG). A correct glycosylation of the protein is crucial for the correct binding of the receptor, the half-life of the hormone and the intended function as such. The exact protein composition of the various commercial PMSG products and their specific glycosylation pattern have not been characterized so far. Therefore, in this study, we used proteomic approaches to analyze and compare four PMSG products. Here we show that the examined PMSGs share a surprisingly low level of commonalities (5.5%) in protein composition among the four tested products, with serum proteins as the main variable. Analysing the site-specific N-glycosylation, we identified for the first time the presence of O-acetylation of sialic acid at the structure of the glycan of eCG, which we could not find in significant amounts on human CG suggesting that this modification is species-specific.