The measurement of nuchal translucency with transabdominal and transvaginal sonography—success rates, repeatability and levels of agreement

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Abstract
The objective of this prospective observational study was to assess the abilities of transabdominal sonography (TAS) and transvaginal sonography (TVS) to measure nuchal translucency (NT) and to establish the levels of agreement between the two scan mode measures. The study was carried out in the Fetal Medicine Unit and 242 pregnant women were recruited from a routine dating scan clinic. All patients underwent TAS and TVS, and two separately obtained measurements of the NT were recorded. The ability to measure NT and the repeatability and levels of agreement between the scan mode measures was assessed. A measurement of NT was obtained in 92% and 90% of fetuses using TAS and TVS, respectively, and in 100% of fetuses by combination of the scan modes. Failure to measure NT with TAS was due to inability to obtain mid-sagittal views because of unfavourable fetal lie or poor resolution, especially in patients with high body mass indexes. Although there was generally good correlation between the two scan mode measurements, a significant overmeasuring of NT was observed with TAS (mean difference ± SD: 0.10 ± 0.29 mm, p<0.01). The repeatability coefficients were 0.40 mm and 0.22 mm for TAS and TVS, respectively. An examiner needs to have the ability and equipment to perform TAS and TVS to consistently measure NT. Considering the overmeasurement of NT with TAS and the improved repeatability with TVS, measurements of NT in a screening programme for chromosomal abnormalities should be by TVS, at least when a prior TAS reveals a value close to a threshold level.