Twins are at greater risk of death and severe morbidity than singletons which is in excess of that attributable to their greater prematurity. Monozygous, specifically monochorionic, twins are at greater risk than dichorionic twins. The major morbidity is neurological impairment usually presenting as cerebral palsy or severe learning disability and frequently, but not always, associated with fetal death of a co-twin. The likely pathogenesis of the neurological impairment is ischaemia attributable to haemodynamic imbalance via placental vascular anastomoses. In addition to the neurological impairment, congenital cardiac, renal, intestinal and other anomalies are more common but discordant in monozygous twins. It has been hypothesized that cerebral palsy and other neurological impairment in apparently singleton infants is attributable to early loss of a twin, the 'vanishing' twin phenomenon. It is also postulated that other congenital anomalies in singletons may be attributable to the same phenomenon.