[The importance of the prenatal life period].

This paper describes the bi-directional influences or transactions between four levels (genetic activity, neural activity, behaviour and environment) that lead to the phenotypical variation, in a probabilistic way. We give an overview of the current scientific knowledge about these transactions in the prenatal life period and describe the inherent risks. Prenatal development is briefly described. We then consider the mechanism of genetic expression and the relationship between genotype and phenotype. Finally a description of recent developments and research findings is given of the discipline of behavioural teratology, which is related to the discipline of developmental psychopathology.