Increased plasma and/or placenta serotonin content are associated with placenta pathology and IUGR in humans and animal models: idiopathic IUGR in humans (10, 11, 27); serotonin or serotonin precursor treatment in rodents (28, 29); SSRI treatment in humans (4, 6, 15), mice (30–32), and sheep (present study); SERT null mouse model (33, 34). This evidence concerns the gene SLC6A4 and fetal growth restriction.