Nuchal translucency cannot be used as a screening test for chromosomal abnormalities in the first trimester of pregnancy in a routine ultrasound practice.

We decided to assess the practicability of introducing nuchal translucency (NT) measurements as a screening programme for fetal Down's syndrome in the first trimester of pregnancy, within the population of women who receive ultrasound examinations in our department. Over a 1-year period, measurements were made in 923 fetuses at < or = 13 weeks' gestation. Fifty-two per cent of the mothers were 36 years or older or had a past history of a chromosomally abnormal fetus or child. Measurements were only successful 58 per cent of the time; this improved to 74 per cent if the fetus was > or = 10 weeks' gestation. Inter-observer variability did not cause a major problem. There were 36 fetuses with an NT > or = 3 mm. Two of these fetuses had a chromosomal abnormality (both trisomy 21). The translucency in these two cases was so large that they would have been detected and offered prenatal diagnosis even prior to this study. There was a total of ten aneuploidies in the study group. Only two of these fetuses were detected by this screening method; five had an NT measurement < 3 mm and in three fetuses (all trisomy 21), measurements were not successful. We outline the practical problems that could be expected by introducing ultrasound screening in a routine setting. Although the efficacy of the test in a research setting may seem good, the effectiveness in everyday usage appears much less impressive, making its uptake as a screening technique in a general ultrasound practice at this stage imprudent.