Image magnification does not contribute to the repeatability of caliper placement in measuring nuchal translucency thickness.

The objective of this study was to assess whether nuchal translucency image magnification contributes to the repeatability of caliper placement. Twenty-seven women undergoing first-trimester ultrasound screening at 11-14 weeks were examined by two qualified examiners. Fetal nuchal translucency thickness was measured twice by each examiner on a regular-sized image and subsequently on the same still image magnified. Intraobserver repeatability coefficients of the regular-sized images were 0.28 mm and 0.34 mm for examiners A and B, respectively. Identical intraobserver repeatability coefficients of 0.28 mm were attained for both examiners on the magnified images. Interobserver repeatability of regular-sized image measurements showed a significant difference (p < 0.01) but the mean difference (+/- standard deviation) of 0.10 (0.18) mm was negligible. Similarly, interobserver repeatability of the magnified-image measurements yielded a significant difference (p < 0.02), but again with a small mean difference of 0.13 (0.26) mm. There were also significant differences when regular-sized images and magnified images were compared for both examiners: 0.08 (0.16) mm for examiner A (p < 0.02) and 0.10 (0.17) mm for examiner B (p < 0.01). Our results demonstrate that nuchal translucency image magnification does not contribute to the reproducibility of the measurement. Despite significantly smaller mean values obtained from the magnified images, compared to the regular-sized measurements, those differences do not justify modification of the criteria for caliper placement on magnified images. Blind repeated measurements on a regular-sized and/or magnified image are recommended as a tool for self-assessment, quality control and training.