Objective To evaluate the feasibility of measuring first trimester nuchal translucency in an unselected population, to assess the relationship with gestation and maternal age and to measure reproducibility. Design A prospective observational study. Setting University College Hospital, London. Subjects One thousand and four women attending for a routine first trimester dating scan between eight and thirteen weeks of gestation. Measurements of nuchal translucency were attempted in 1368 (80.3%) and successful in 1127 (82% of attempts). Results Nuchal translucency is most easily measured at 11 weeks of gestation. If a cut-off of greater than or equal to 3 mm is used, 6% of unselected fetuses between eight and thirteen weeks of gestation are classified as abnormal. Nuchal translucency increases with gestational but not maternal age. Reproducibility is poor: by repeating measurements with a different operator, the same operator using a different still image, or the same operator using the same still image, 18.8%, 17.5% or 12.4% of nuchal translucency measurements, respectively, change their classification as normal or abnormal. Conclusions If nuchal translucency greater than or equal to 3 mm were used as an indication for karyotyping, 6% of the normal pregnant population would be screen positive. However, the percentage will vary greatly depending on the gestational age profile of the screened population. The poor reproducibility of nuchal translucency measurement could diminish its usefulness as a screening test for Down's syndrome.
