[Diagnosis of trisomy 21: nuchal translucency and/or serum markers?]

To compare alternative methods of antenatal screening for Down's syndrome (nuchal translucency measurement and second trimester maternal serum screening) in a low-risk population and to evaluate the consequences of a sequential estimation of risk.

In a consecutive series of 4308 women aged less than 38 with a singleton pregnancy, we examined the detection rate of nuchal translucency (NT) measurement at 10-14 weeks and maternal serum screening (MSS) by human chorionic gonadotropin and alpha-feto-protein at 14-18 weeks. Women with a NT measurement = 3 mm and women with a MSS derived risk = 1/250 were recommended to have an amniocentesis. A second trimester detailed ultrasound scan was also performed in all women. The outcome of all pregnancies was entered in a computerized database and the detection rate and false-positive rate of different screening strategies were analysed.

Of the 4308 pregnancies that were followed (mean maternal age 30.1 years), there were 12 cases of Down's syndrome (0.28%), all detected prenatally. Seven of twelve cases had a NT measurement above 3 mm (58%), and 6 out of ten cases with available MSS had a calculated risk = 1/250 (60%). Four of the five cases with NT measurement below 3 mm were detected by subsequent MSS. At a threshold giving 5% of positive tests, the sensitivity of NT screening and MSS were 75% and 60%, respectively.

Sequential screening for Down's syndrome by nuchal translucency and second trimester biochemistry is effective and appears to increase the detection rate compared to the use of any single test. However, this strategy is likely to raise the false-positive rate and the interpretation of MSS derived risk should be combined to the first trimester NT measurement.