Abstract In equids, placentation is diffuse and the nutrient supply to the fetus is determined by uterine size, which is correlated to maternal size. The size of the mare affects fetal development [Allen WR et.al..Reproduction 2002;123(3):445-53]and subsequent postnatal growth [Allen WR et.al. Reproduction 2004;127(1):67-77]. We induced intrauterine growth enhancement or restriction through embryo transfer using Pony (P), Saddlebred (S) and Draft (D) horses and investigated growth measurements and hormones from birth to one year of age. Control P-P (n=21) and S-S (n=28) pregnancies were obtained by artificial insemination. Enhanced and restricted pregnancies were obtained by transferring P (P-D, n=6) and S embryos (S-D, n=8) into D mares or S embryos into P mares (S-P, n=6). From birth to weaning (180 days), control and experimental foals were raised by their dams and recipient mothers, respectively. Body weight, chest circumference, withers' height, front leg length, hip and shoulder widths were recorded from day 3 to 380. Plasma T3 and IGF1 were assayed on days 3, 90, 180, and 380. S-S were heavier and had larger body measurements and lower IGF1 and T3 concentrations compared to P-P from day 3 to 380 (p<0.05). Fetal growth was not enhanced in S-D, body measurements and hormonal concentrations at birth being similar to S-S. Weight gain from day 3 to 140, however, was faster in S-D versus S-S (p<0.05). Other growth parameters were not affected and weight gain became undistinguishable from S-S after weaning. Fetal growth was enhanced in P-D: at birth, they were 57% heavier and had overall increased body measurements compared to P-P (p<0.05). Body weight, chest circumference, withers' height and front leg length remained larger until day 380 (p<0.05), with T3 concentrations being reduced compared to P-P (p<0.05). Fetal growth was restricted in S-P: they were 37% lighter and had overall restricted body measurements compared to S-S (p<0.05). Body weight, chest circumference, withers' height and front leg length remained smaller until weaning (p<0.05) and were associated with reduced T3 concentrations compared to S-S (p<0.05). Neither the restricted nor the enhanced fetal environment affected IGF1 concentrations. In conclusion, in the present model, the combined effects of enhanced fetal and postnatal growth persisted until 18 months of age, whereas catch up growth appeared to occur postweaning in restricted foals.