Effect of first-trimester nuchal translucency on second-trimester maternal serum biochemical screening for Down's syndrome

We evaluated the effectiveness of 10-14 week nuchal translucency measurement in routine ultrasound screening for Down's syndrome, and its effect on the sensitivity of subsequent maternal serum biochemistry. This was an observational study, in which all women attending for antenatal care at a district general hospital were routinely offered a first-trimester nuchal translucency scan and second-trimester quadruple maternal serum biochemistry as screening tests for Down's syndrome. The main outcome measures were abnormal fetal karyotype and the performance of screening tests. A total of 3604 women were scanned in the first trimester of pregnancy. Excluding the cases that did not fit the entry criteria (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 pregnancy 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-positive rate of 7.5% (n = 143). Only one extra case of Down's syndrome would 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 measurement is effective in routine screening for fetal chromosomal abnormality. Furthermore, the implementation of a nuchal translucency screening program will significantly reduce the positive predictive value of second-trimester maternal serum biochemistry testing.