Although DNA replication is a very accurate process, a small number of new mutations are generated at every cell division. The generation of a new mutation during the formation of an ovum or sperm cell can cause an early miscarriage or birth defect. The generation of new mutations during embryogenesis can cause a variety of localized birth defects. The molecular delineation of these errors in somatic and gonadal cells has clarified the basis of some birth defects, and has both refined and complicated genetic counselling for a number of paediatric conditions. The processes responsible for these new mutations are present in all cells. For this reason new mutations accumulate in all cells throughout life and contribute to the ageing process. Thus the molecular events that cause many miscarriages and birth defects are the same as those that ultimately lead to death.