UK MULTICENTER PROJECT ON ASSESSMENT OF RISK OF TRISOMY-21 BY MATERNAL AGE AND FETAL NUCHAL-TRANSLUCENCY THICKNESS AT 10-14 WEEKS OF GESTATION

Background Prenatal diagnosis of trisomy 21 currently relies on assess ment of risk followed by invasive testing in the 5% of pregnancies at the highest estimated risk. Selection of the high-risk group by a comb ination of maternal age and second-trimester maternal serum biochemist ry gives a detection rate of about 60%. We investigated assessment of risk by a combination of maternal age and fetal nuchal-translucency th ickness, measured by ultrasonography at 10-14 weeks of gestation. Meth ods The risk of trisomy 21 was estimated for 96 127 women of median ag e 31 years (range 14-49) with singleton pregnancies. Ultrasonography w as done by 306 appropriately trained sonographers in 22 centres, Risk of trisomy 21 was calculated from the maternal age and gestational-age -related prevalence, multiplied by a likelihood ratio depending on the deviation from normal in nuchal-translucency thickness for crown-rump length. The distribution of risks was investigated and the sensitivit y of a cut-off risk of 1 in 300 was calculated. Phenotype was assessed by fetal karyotyping or clinical examination of liveborn infants. Fin dings The estimated trisomy-21 risk, from maternal age and fetal nucha l-translucency thickness, was 1 in 300 or higher in 7907 (8.3%) of 95 476 normal pregnancies, 268 (82.2%) of 326 with trisomy 21, and 253 (7 7.9%) of 325 with other chromosomal defects. The 5% of the study popul ation with the highest estimated risk included 77% of trisomy-21 cases , Interpretation Selection of the high-risk group for invasive testing by this method allows the detection of about 80% of affected pregnanc ies, However, even this method of risk assessment requires about 30 in vasive tests for identification of one affected fetus.