The aetiology of spastic cerebral palsy (CP), in the majority of cases, is not known but the general consensus is that cerebral impairment occurs prepartum. In monochorionic twin pregnancies, death of one twin late in gestation is recognised as being an important risk factor for the surviving cotwin to have CP. It has been suggested that a significant proportion of singletons with spastic CP may be the result of death of a cotwin in the second half of gestation. In this paper it is hypothesised that spastic CP of unknown aetiology is the result of the death of a monochorionic cotwin and that the death of the cotwin may impair the neurological development of the survivor throughout gestation. If so, vanishing-twin syndrome, which is now a recognised phenomenon revealed by ultrasound examination in early pregnancy, is important in the aetiology of spastic CP.