Down’s syndrome (DS) is one of the most common birth defects. It is a sentinel phenotype which can be easily recognized and cytogenetically verified. Most cases are due to maternal non-disjunction. Obviously, oogenesis is an error-prone process due to the lack of a chromosome-mediated checkpoint control. It is, therefore, sensitive to the effect of endogenous and exogenous factors, especially around conception. This article will focus on risk factors for trisomy 21. Its aim is not to give a comprehensive re­view but a brief, exemplary selection. It is concluded that surveillance of trisomy 21 is particularly suitable for epi­demiological studies with respect to environmental haz­ards that might influence the rate of non-disjunction. This is essential from a public health perspective under the aspect of risk avoidance and primary prevention.

Key words: Trisomy 21; Non-disjunction; Ionizing radiation; Down’s syndrome (DS).