EFFECT OF FIRST-TRIMESTER NUCHAL TRANSLUCENCY ON 2ND-TRIMESTER MATERNAL SERUM BIOCHEMICAL SCREENING FOR DOWNS-SYNDROME

We evaluated the effectiveness of 10-14 week nuchal translucency measu rement in routine ultrasound screening for Down's syndrome, and its ef fect on the sensitivity of subsequent maternal serum biochemistry. Thi s was an observational study, in which all women attending for antenat al care at a district general hospital were routinely offered a first- trimester nuchal translucency scan and second-trimester quadruple mate rnal serum biochemistry as screening tests for Down's syndrome. The ma in outcome measures were abnormal fetal karyotype and the performance of screening tests. A total of 3604 women were scanned in the first tr imester of pregnancy. Excluding the cases that did not fit the entry c riteria (n = 344, 9.6%) and in which nuchal translucency measurements were not possible (n = 340, 9.4%), a total of 2920 women were screened . A nuchal translucency-derived risk of 1 : 200 for an aneuploid pregn ancy resulted in a 5% (n = 147) screen-positive rate. Screening using this risk enabled the first-trimester detection of five of seven (71%) fetuses with trisomy 21 and 14 of 18 (78%) aneuploid fetuses. Second- trimester maternal serum biochemistry testing was performed in 2904 of the women who had nuchal translucency screening, with a screen-positi ve rate of 7.5% (n = 143). Only one extra case of Down's syndrome woul d have been detected by maternal serum biochemistry testing if nuchal translucency screening had been implemented at a risk level of 1 : 300 . This study demonstrates that first-trimester nuchal translucency mea surement is effective in routine screening for fetal chromosomal abnor mality. Furthermore, the implementation of a nuchal translucency scree ning program will significantly reduce the positive predictive value o f second-trimester maternal serum biochemistry testing.