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Arginine is an important amino acid for angiogenesis and vasodilation, and recent studies have established ideal amino acid ratios for maternal, placental, fetal, and mammary gland tissue growth during gestation. This study was carried out to evaluate the effects of supplementing gestation diets with arginine and/or amino acid blend (lysine, methionine, threonine, and tryptophan) on placental efficiency and piglet birth weight (BW) in hyper-prolific females. Pregnant gilts were divided into four treatment groups, namely, Control (corn-soybean meal based diet from D25 to D112), Arginine (supplemented with 1% Arg from D25 to D80); Blend (20 g of blend from D81 to D112) and Argiblend (supplemented with 1% Arg from D25 to D80 and 20 g of blend from D81 to D112 of gestation). The supplementation with Blend increased the weight of gilts at D112). The total number of piglets born, percentage of mummified fetuses, average BW, within-litter coefficient of variation in BW, percentage of low-birth-weight piglets (≤850 g or ≤1000 g), placental weight, placental efficiency and expression of vascular endothelial growth factor (VEGF) were not affected by the supplements. The supplementation with arginine decreased the number of born alive piglets and increased then percentage of stillborn piglets. The average placental weight was higher in Blend than in Control gilts. The effects of supplementation were also evaluated based on prolificacy of females categorized as either high (>14 total piglets born) and low (≤14 total piglets born). Within LowProlif group, gilts fed on the blend treatment had increased LitBW and AvgPBW compared to control gilts. LowProlif gilts supplemented with arginine also had higher AvgPBW than Control gilts. The LowProlif gilts fed on the arginine or blend diets had lower percentage ( P 0.05) of low BW piglets (≤850 g and ≤1000 g) than those fed on the control and argiblend diet. Even though gestation diets supplemented with arginine and/or an amino acid blend did not influence average piglet BW and the within-litter variation in BW, when all the litters were taken into account, the supplementation with arginine or blend increased the average BW and reduced the percentage of low-weight piglets at birth when the litter size was less than 14.