This study aimed to investigate the effects of silymarin supplementation on sow health, reproductive performance, colostrum quality, immune function, and blood biochemistry during late pregnancy. Twenty parity-4 sows (Landrace × Yorkshire) were included in the study and randomly assigned within each parity group to a control diet (CG, basic diet, n = 10) and a Silymarin diet (SIL, basic diet + 0.2 g/kg SIL, n = 10), from the 85th day of gestation until weaning. Blood and milk samples were collected for analysis. The results demonstrated that silymarin supplementation significantly reduced the number of stillbirths and the stillbirth rate (P <
0.05). Additionally, silymarin supplementation tended to increase the mean body weight of newborn and healthy piglets (P = 0.10
P = 0.08) and improved the intrauterine growth restriction (IUGR) score (P = 0.07). Silymarin supplementation also significantly increased protein content in sow colostrum (P <
0.05) and showed a trend towards higher total solids and lactose content (P = 0.05
P = 0.09). Furthermore, silymarin supplementation significantly increased immunoglobulin G (IgG), IgA, and IgM levels in sow colostrum (P <
0.05), as well as proline content (P = 0.10). It also significantly increased endothelial nitric oxide synthase (eNOS) levels in umbilical cord blood (P <
0.05). Moreover, silymarin supplementation reduced interleukin-6 (IL-6) levels and increased interleukin-10 (IL-10) levels in umbilical cord blood serum (P <
0.05). In conclusion, silymarin supplementation in late-gestation sow diets has no adverse effects on sow or piglet health. Instead, it promotes placental vascular development, reduces inflammation, and decreases stillbirth and IUGR rates. It also enhances the average birth weight of viable piglets and modulates the composition of sow colostrum, enriching key nutrients, immunoglobulins, and amino acids.