The objective of this study was to test whether a high inclusion rate of dietary vitamin B₂, B₆, B₁₂, and folic acid during the first 45 d of gestation could decrease within-litter variation in piglet birth weight and increase the average birth weight of the litter. Two hundred forty-one multiparous sows (Landrace x Yorkshire) were allotted to either a control or a B vitamin diet from mating to d 45 of gestation. The experimental unit was the sow or the litter. Sows in the control group were fed a standard gestation diet complying with Danish recommendations: 5.11, 3.07, 0.02, and 1.53 mg/kg for vitamin B₂, B₆, B₁₂, and folic acid, respectively. Sows fed the B vitamin diet were fed the same standard gestation diet including an extra premix providing 232, 210, 1000, and 668 % extra vitamin B₂, B₆, B₁₂, and folic acid, respectively. From d 46 until farrowing, all sows were fed the standard diet. The backfat thickness of sows was measured at farrowing and weaning in the previous cycle and at d 14, 45, and 112 of gestation. From a subsample of 10 second parity sows per treatment, blood samples were taken using jugular venipuncture into 10 mL EDTA tubes at weaning and d 14, 45, and 112 of gestation. The plasma was analyzed for the concentration of homocysteine. At farrowing, the number of live and stillborn piglets were recorded, and for a subsample of 71 (control) and 66 (B vitamin) sows, the weight, crown-rump length and abdominal circumference of individual piglets in the litters were measured. The plasma concentration of homocysteine at weaning and during gestation was unaffected by dietary treatments. For the overall treatments, there was no difference in live born (Control: 19.5; B vitamin: 19.9) and stillborn (Control: 1.5; B vitamin: 1.6) piglets between treatments. The average birth weight of live born (Control: 1,270 g; B vitamin: 1,245 g) and stillborn (Control: 992 g; B vitamin: 870 g) piglets and the within-litter variation in birth weight (Control: 294 g; B vitamin: 307 g) were unaffected by dietary treatment. The abdominal circumference (Control: 23.3 cm; B vitamin: 22.7 cm) and the crown-rump length (Control: 31.6 cm; B vitamin: 31.0 cm) were greater for piglets born to sows fed the control diet compared to sows fed the B vitamin diet (P < 0.05). In conclusion, there was no positive effect of supplementing surplus B vitamins above recommended inclusion rates to sows in early gestation on piglet birth weight.