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Reproducibility of the Fetal Nasal Bone Length Measurement

Objective. To investigate the reproducibility of the fetal nasal bone length measurement in the first trimester. Methods. In this prospective study, crown‐rump length, nuchal translucency, and nasal bone length were measured in 90 singleton pregnancies between 11 and 14 weeks of pregnancy. The study was divided into 2 equal substudies. Three investigators measured the nasal bone in study I. After an intensive 3‐month training program, the same 3 investigators measured the nasal bone in study II. Measurement of the nasal bone was standardized. All investigators were blinded and measured the nasal bone 3 times. The intraclass correlation coefficient (intra‐CC) and interclass correlation coefficient (inter‐CC) were calculated. Results. Nasal bone length measurement was successfully performed in all cases. The median nasal bone lengths were 2.3 (range, 1.5–3.2), 2.6 (range, 1.4–4.2), and 2.9 (range, 2.1–3.8) mm between 77 and 83, 84 and 90, and 91 and 98 days' gestational age, respectively; median crown‐rump length was 63.0 (range, 45.0–83.3) mm; and median nuchal translucency measurement was 1.3 (range, 0.9–2.5) mm. The correlation between the 3 observers was poor (inter‐CC, 0.32) in the first study. After the training program, the interobserver correlation improved but was still moderate (inter‐CC, 0.64). In both studies the interobserver variability did not differ between 11, 12, and 13 weeks' gestational age. There was no relationship between the mean nasal bone length and interobserver difference in both studies. The intraobserver variability was good for each investigator in both studies (intra‐CC: study I, 0.93, 0.95, and 0.97; study II, 0.98, 0.97, and 0.97, respectively). Conclusions. This study shows that the reproducibility of the fetal nasal bone length measurement in the first trimester is inadequate.