OBJECTIVES: The purpose of this study is to assess the feasibility of foetal 
nasal bone (NB) measurement during the first trimester of pregnancy, and to 
examine the contribution of this measurement to the prenatal screening for Down 
syndrome following the definition of NB threshold using ROC curves in an 
unselected population.
METHODS: This prospective study was carried out at our centre SIHCUS-CMCO 
(reference centre) from January 2002 to December 2004 on a total of 2,044 
pregnant outpatients at gestational weeks 11-14. Only 1260 singleton foetuses 
were used for statistical analysis. In the 784 other patients, we were unable to 
obtain a correct image allowing a reproducible measurement. NB was measured 
during the same session as nuchal translucency (NT) measurement. Ten trained 
sonographers took part in the study. Correlation index was evaluated to shed 
light on a link between interest variables and NB. Screening values of NB 
measurement in T 21 were also calculated with NB measurement according to 
crown-rump length, and expressed as the best threshold of multiple of the median 
determined by ROC curve. Screening values of genetic ultrasound were then 
evaluated by adding NB measurement to maternal age and NT measurement.
RESULTS: Two thousand and forty-four patients were included. We indexed 30 cases 
of T 21, 14 cases of Trisomy 18, 10 cases of Trisomy 13 and 25 cases of other 
karyotype abnormalities. Feasibility of measurement was 62% of all cases. We 
observed a significant relation between NB and NT (p = 0.001 ), as well as 
between NB and crown-rump-length (p < 0.0001 ). However, size of NB was not 
correlated to maternal ethnic group (p = 0.314). At 0.6 multiple of the median 
thresholds, screening values of NB measurement in T 21 were: sensibility 32%, 
false positive rate 10%, positive predictive value 13.6%, and negative 
predictive value 96.9%. The likelihood ratio for T 21 in case of NB < or = 0.6 
multiple of the median was 4.4 (2.0-9.4). Screening values for maternal age and 
NT measurement were: sensitivity 88%, false positive rate 23%,positive 
predictive value 9.7%, and negative predictive value 99.6%. Inclusion of NB 
measurement increased sensitivity to 100%, positive predictive value to 13.6%, 
and negative predictive value to 100%, and decreased false positive rate to 5%.
CONCLUSION: NB measurement seemed to be a great sonographic marker for T 21. 
However, its low feasibility made it inadequate for routine settings in first 
trimester T 21 screening in an unselected population. Statistical independence 
with NT thickness needed to be further evaluated.