Long-term maternal porcine ST (pST) treatment and greater maternal parity increased litter mean piglet BW after fostering, at d 14 of lactation, and at weaning. Landrace × Large White gilts and sows (parities 2 and 3 at mating) were uninjected (controls) or were injected daily with pST (gilts: 2.5 mg/d; sows: 4.0 mg/d, each approximately 15 μg of pST/kg per day) from d 25 to 50 of pregnancy (short-term pST) or from d 25 to 100 of pregnancy (long-term pST). Fostering was performed on the day of birth to achieve litter sizes at the beginning of lactation of 10 piglets (where possible) in first-lactation sows and 12 piglets in older lactating sows. Lactating pigs were fed 3 times daily to appetite, and the feed offered was recorded daily. Litter size and piglet BW were recorded after the completion of fostering, on the day of birth, on d 14 of lactation, and before weaning. Maternal pST treatment from d 25 to 50 of pregnancy increased mean piglet BW after fostering (P = 0.011), but not at d 14 of lactation (P > 0.5) or at weaning (P > 0.8). Maternal pST treatment from d 25 to 100 of pregnancy increased mean piglet BW after fostering (P ≤ 0.001), on d 14 of lactation (P = 0.037), and at weaning (P = 0.017). Mean piglet BW was greater in litters from sows than those from gilts, after fostering, at d 14 of lactation and at weaning (each P ≤ 0.001).