Screening for trisomy 21 in Flanders: a 10 years review of 40.490 pregnancies screened by maternal serum.

Objective: To evaluate maternal serum screening for trisomy 21 (MSS) in Flanders between 1992 and 2002.

Study Design: Data of a large database on the results of MSS, nuchal translucency (NT) and pregnancy outcome were analysed retrospectively.

Results: Despite an excellent performance of second trimester MSS at a maternal age > or = 35 years (94.4% detection rate (DR) of trisomy 21 at a false positive rate (FPR) of 22.4%), the proportion of patients above 35 years of age in the study population was significantly lower than in the Flemish general pregnant population (5.5% versus 8.9%, P < 0.001). In the population screened by MSS and NT, the DR of second trimester MSS at a 5% FPR was 44.4%, which was lower than 66.6% in the population screened by MSS without NT. When nine trisomy 21-affected pregnancies were compared to 3265 normal pregnancies, the mean NT-MoM values were not significantly different (1.16 +/- 0.89 versus 1.00 +/- 0.46, P > 0.05). Both the findings comply to a sequential screening practice where second trimester MSS is only performed after a normal measurement of NT in the first trimester.

Conclusion: In Flanders, the uptake of second trimester maternal serum screening is low in women aged 35 years or more. Its screening performance decreased after the introduction of sequential screening.