If one takes into account that the prevalence of aneu­ploidies is more than an order of magnitude higher in man than in the mouse, extrapolation from mouse-to-human with respect to the induction of non-disjunction poses a fundamental problem. Thus, there is a need for epidemio­logical studies in man with respect to environmental haz­ards that might influence the rate of non-disjunction or the identification of women with a higher risk for the develop­ment of aneuploid gametes.

For reasons of efficiency (time and money), it is gen­erally desirable to use existing databases. An enormous number of subjects affected with trisomy 21 is continu­ously detected due to the extensive use of prenatal diagno­sis programs. In addition, in several countries all tentative diagnoses of DS in the newborn or infancy period, are confirmed by cytogenetic analyses. These diagnoses can be used as a basis for a case-control surveillance program. The relevant history of the mothers of these cases can be obtained with little extra effort; the challenge for the oper­ation of such a system is to obtain the co-operation of healthy controls. The interviews must be conducted by trained interviewers as personal interviews with a struc­tured questionnaire. The questionnaire should include socio-demographics, family history of the woman and her partner, gynecological, obstetric and general medical history, life style (including smoking, alcohol, drugs), previous X-rays and work-place history, so that all sus­pected confounders and risk factors are covered.

We have shown the feasibility of such an approach and interviewed 50 mothers, who gave birth to a child with trisomy 21, and 272 controls. Since the probability of receiving a particular X-ray investigation increases with age, the analysis was stratified using the Mantel-Haenszel method. The highest (and significant) odds ratios were for thyroid scans, and for X-ray investigations of the pelvis and the abdomen [26]. This is in line with the BEIR V (Committee on the Biological Effects on Ionizing Radia­tion) report on the effects of low dose radiation on non-disjunction in humans. The authors conclude that nine out of 13 studies show a positive, and only two a negative effect [27].

In conclusion, there is an urgent need for epidemiolog­ical studies of trisomy 21. This is essential from a public health perspective under the aspect of risk avoidance and primary prevention. Knowledge of potential hazards is a prerequisite for the practitioner or clinician who counsels and takes care of pregnant women according to the rules of good clinical practice.