Examination of fetal nasal bone and repeatability of measurement in early pregnancy.

OBJECTIVE

To evaluate the examination and measurement of fetal nasal bone at 10-14 weeks of gestation.

METHODS

The study included 501 fetuses in 496 consecutive pregnant women attending for the routine first-trimester ultrasound examination. The presence or absence of the fetal nasal bone was determined in the mid-sagittal plane and the length was measured by one of four examiners (measurement A; n = 501). A second measurement was taken by the same examiner (B, n = 300) and a different examiner repeated the measurement (C, n = 200) whenever possible. Measurements were made to the nearest 0.1 mm. The duration of one hundred consecutive examinations was recorded, as was that of another 100 consecutive routine first-trimester examinations without measuring the nasal bone.

RESULTS

The median nasal bone length was 1.6 (0.8-2.4) mm, the median gestational age was 12 (10-14) weeks and the median crown-rump length (CRL) was 63 (32-90) mm. The fetal profile was examined in all 501 cases and the fetal nasal bone was present in all but one case (99.8%). No transvaginal scans were needed for the examination of nasal bone only. The average time for the sonographic examination (8.3 min) was not significantly different from the average time for first-trimester scans in which the fetal nasal bone was not measured (8.0 min). The fetal nasal bone length increased from 1.1 mm at a CRL of 35 mm to 2.1 mm at a CRL of 90 mm (nasal bone = 0.016 x CRL + 0.619, P < 0.001, r = 0.655). The repeatability coefficient for intraobserver measurements was 0.080 mm and the coefficient for interobserver measurements was similar (0.083 mm).

CONCLUSIONS

The nasal bone can be detected from 10 weeks of gestation onwards. Consistent visualization and repeatable measurement of fetal nasal bone can be performed by an experienced sonographer in the first trimester without extending the length of time required for scanning or introducing the need for transvaginal sonography.