BioThrive supplement is a proprietary blend of plasma fractions including serum-based bioactive proteins. The objective of this study was to evaluate the effect of dosing BioThrive of porcine origin to mares on serum metabolites and stress biomarkers of mares and their foals during late pregnancy, at parturition, and at 30 and 60 days of lactation. In addition, specific reference values for serum parameters of mares and their foals were reported. Pregnant mares (n = 26) housed in two barns, Ask (n = 11) and Blue Hors (n = 15) were assigned to either a placebo (n = 13) or BioThrive (n = 13) supplement (240 grams per day) that was divided into two daily doses starting on average 55 days before foaling until approximately 60 days after foaling. Mare body weight (BW) was recorded at −5, 5, 30, and 60 days of parturition. Serum samples from mares were collected prepartum at an average of 55 days before and 5, 30, and 60 days after parturition. Foal BW was recorded, and serum samples were collected from foals at birth, 30, and 60 days of age. Mares given BioThrive had a tendency for lower (P = .069) BW at birth and had a tendency (P = .072) for less TNF-α during pregnancy and higher IgG at foaling (P = .020). Foals from mares given BioThrive supplement had a tendency for reduced TNF-α at foaling (P = .097). Results may suggest that BioThrive had some effect on certain stress biomarkers in mares fed with this supplement.