Sialic acids (Sia) are key monosaccharide constituentsof sialylated glycoproteins (Sia-GP), human sialylatedmilk oligosaccharide (Sia-MOS), and gangliosides.Human milk sialylated glycoconjugates (Sia-GC) arebioactive compounds known to act as prebiotics andpromote neurodevelopment, immune function, and gutmaturation in newborns. Only limited data are availableon the Sia content of porcine milk. The objectiveof this study was to quantitatively determine the totallevel of Sia N-acetylneuraminic acid (Neu5Ac), Nglycolylneuraminicacid (Neu5Gc), and ketodeoxynonulosonicacid (KDN) in porcine milk and to comparethese levels in gilt and sow milk during lactation. Milkfrom 8 gilts and 22 sows was collected at 3 stages of lactation(colostrum, transition, and mature milk). Standardand experimental samples were derivatized using1,2-diamino-4,5-methylenedioxy-benzene and analyzedby ultra-high-performance liquid chromatography usinga fluorescence detector. The following new findingsare reported: (1) Gilt and sow milk contained significantlevels of total Sia, with the highest concentrationin colostrum (1,238.5 mg/L), followed by transitionmilk (778.3 mg/L) and mature milk (347.2 mg/L); (2)during lactation, the majority of Sia was conjugatedto Sia-GP (41'46%), followed by Sia-MOS (31'42%)and a smaller proportion in gangliosides (12'28%); (3)Neu5Ac was the major form of Sia (93'96%), followedby Neu5Gc (3'6%) and then KDN (1'2%), irrespectiveof milk fraction or stage of lactation; (4) the concentrationof Sia in Sia-GP and Sia-MOS showed a significantdecline during lactation, but the level of gangliosideSia remained relatively constant; (5) mature gilt milkcontained a significantly higher concentration of Sia-GP than sow milk. The high concentration of totalSia in porcine milk suggests that Sia-GC are importantnutrients that contribute to the optimization ofneurodevelopment, immune function, and growth anddevelopment in piglets. These findings provide an importantrationale forthe inclusion of Sia-GC in pig milkreplacers to mimic porcine milk composition for theoptimal growth and development of piglets.