These in-vitro studies are supported by experiments in healthy guinea-pigs, in which maternal IGF-1 infusion in mid-pregnancy increased placental transport via the system A transporter (slc38a2) in both mid and late pregnancy,and by studies in growth-restricted ovine fetuses, in which intra-amniotic IGF-1 treatment in late pregnancy increased fetal growth and placental mRNA expression of the system A (slc38a4) and L (slc7a8) amino acid transporters.