[Epidemiological evaluation: the special case of prenatal diagnosis].

Two procedures of prenatal diagnosis were the subject of epidemiological evaluations. The first one is the screening of Down syndrome among pregnant women 38 years of age and over. The other is the antenatal screening of major malformations by ultrasound. An analysis of the different studies published is presented. The interventions are defined and the frequencies of pathologies involved are given. A review of risks, efficacy and effectiveness of each intervention was then carried out. For Down syndrome screening, the criterion for efficacy and effectiveness is the case prevented. Maximum expected efficacy if the attendance of pregnant women 38 years and over to amniocentesis was 100% is 21%. In practice, the effectiveness (24%) is higher than the efficacy (21%) for the Bouches du Rhône district because health care channels to amniocentesis are not based only on the indication of maternal age. During these last three years in the Bouches du Rhône area, 25% of the trisomies 21 detected were so because amniocentesis was carried out after an abnormal sonographic sign had been detected, and 75% because of maternal age. The limitations of the effectiveness are detailed. For the ultrasound screening of major malformations, the efficacy and the effectiveness are assessed through the intermediate criterion of "number of antenatal diagnoses performed". The overall sensitivity of screening in the general population varied between 55% and 75% according to the studies but it was only between 28% and 39% at the 24th week of gestation. The most often screened anomalies were of the central nervous system and the least often screened were congenital cardiopathies. The two interventions were then compared for their objectives and limitations.