Nasal bone assessment in fetuses with trisomy 21 at 16-24 weeks of gestation by three-dimensional ultrasound.

OBJECTIVE: This study aims to investigate the length and shape of the nasal bones in fetuses with trisomy 21 at 16-24 weeks' gestation.
METHOD: We acquired three-dimensional volumes of the fetal profile from 41 fetuses with trisomy 21. The multiplanar mode was used to measure nasal bone length in the exact midsagittal plane and in parasagittal and oblique views of the fetal face and to examine the nasal bones in the coronal plane.
RESULTS: There was bilateral absence of the nasal bones in 11 (26.8%) cases and unilateral absence in one (2.4%). In 29 (70.7%) cases with present nasal bones, there was progressive over-estimation of nasal bone length when measured in parasagittal and oblique views compared to measurements taken in the exact midsagittal plane. In the coronal plane, in 18 of 29 (62.1%) fetuses with trisomy 21, the nasal bones were divergent, whereas in 131 of 135 (97.0%) euploid fetuses, the bones were entirely fused in the midline.
CONCLUSION: Parasagittal and oblique scanning planes may produce over-estimation of nasal bone length in trisomy 21 fetuses because they often have divergent nasal bones. Consequently, it is essential that measurement of nasal bone length is carried out in the exact midsagittal plane of the face.