[Prenatal diagnosis of trisomy 21: the Tunisian experience].

Unlabelled: Down syndrome (Trisomy 21) is the most common fetal chromosomal abnormality in humans. Its clinical signs are now well known. Methods for prenatal screening have advanced substantially in the past two decades.

Objective: To describe our experience with prenatal diagnosis of Down syndrome, including the indications, methods and results.

Material And Methods: This retrospective study examined cases over a 4-year period. We adopted a sequential screening strategy for patients followed in our department since the beginning of their pregnancies after informed consent. We proposed first trimester ultrasound that measured nuchal translucency thickness and followed it with maternal serum screening. Some patients underwent screening during the second trimester or third trimester ultrasound. To assess the results, we studied the mothers' epidemiological characteristics and analysed the circumstances of prenatal diagnosis of trisomy 21 (T21).

Results/discussion: We identified 22 cases of T21 during the study period, for a total prevalence of 0.98 per thousand. The diagnosis was prenatal in 13 cases, mainly due to ultrasound signs. Of the 14 patients seen prenatally, only 8 were followed from early pregnancy. Five had enlarged nuchal translucency (> 95th percentile). Three had positive second trimester serum screening tests. One patient had amniocentesis planned because of her age (table 1). T21 was diagnosed in the second trimester in two cases and in the third trimester in three. The major morphological abnormalities observed were cardiac. We found an atrioventricular canal defect in four cases, and fetal hydrops in two cases (Table 2). The median gestational age at diagnosis of T21 in this study was 21 weeks. The diagnosis was missed in one patient followed throughout pregnancy in our unit. The median gestational age at termination of pregnancy was 22 weeks. Only one patient chose not to terminate the pregnancy. Her fetus, delivered at term, had no major pathologies.

Conclusion: The establishment of a screening strategy for trisomy 21 in Tunisia is necessary to reduce handicaps. It should begin by expanding first-trimester ultrasound with nuchal translucency measurement. At the same time, serum marker testing should be offered to all patients. Routine amniocentesis for advanced maternal age should be avoided.