Fetal growth is determined by fetal genotype and, to a large extent, by the uterine environment. The in utero environment is predominantly influenced by maternal genetics, the size of the mother, the capacity of the placenta to provide nutrients to the fetus, and, to a modest degree, paternal genetics. Although several genes have been described as maternally or paternally imprinted, two growth disorders, Beckwith-Wiedemann syndrome (macrosomia) and Russell-Silver syndrome (fetal malnutrition), represent well-characterized phenotypes that arise as a consequence of disrupted imprinting. Two protein products of genes, such as insulin-like growth factors I and II, play a specific role in the growth of trophoblastic cells, which form the placenta.[1] In a genetically abnormal fetus, genes affecting growth may result in adverse effects. Various factors affecting fetal growth are listed in Table 97-1.