The objective of this study was to test whether a high inclusion rate of dietary vitamin B2, B6, B12, and folic acidduring the first 45 d of gestation could decrease within-litter variation in piglet birth weight and increase theaverage birth weight of the litter. Two hundred forty-one multiparous sows (Landrace x Yorkshire) were allottedto either a control or a B vitamin diet from mating to d 45 of gestation. The experimental unit was the sow or thelitter. 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 dietwere fed the same standard gestation diet including an extra premix providing 232, 210, 1000, and 668 % extravitamin 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, and112 of gestation. From a subsample of 10 second parity sows per treatment, blood samples were taken usingjugular venipuncture into 10 mL EDTA tubes at weaning and d 14, 45, and 112 of gestation. The plasma wasanalyzed for the concentration of homocysteine. At farrowing, the number of live and stillborn piglets wererecorded, and for a subsample of 71 (control) and 66 (B vitamin) sows, the weight, crown-rump length andabdominal circumference of individual piglets in the litters were measured. The plasma concentration of ho-mocysteine 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) andstillborn (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; Bvitamin: 22.7 cm) and the crown-rump length (Control: 31.6 cm; B vitamin: 31.0 cm) were greater for pigletsborn to sows fed the control diet compared to sows fed the B vitamin diet (P < 0.05). In conclusion, there was nopositive effect of supplementing surplus B vitamins above recommended inclusion rates to sows in early gestationon piglet birth weight.