Assessment of the gap between the fetal nasal bones at 11 to 13 + 6 weeks of gestation by three-dimensional ultrasound.

OBJECTIVE To detect the presence of a gap between the fetal nasal bones at 11 to 13 + 6 weeks of gestation and to verify if this gap could lead to the erroneous diagnosis of absent nasal bone. METHODS Three-dimensional (3D) ultrasound was used to assess the fetal nose in 450 singleton pregnancies, immediately after two-dimensional (2D) evaluation of the nasal bones and screening for chromosomal defects by a combination of maternal age and the measurement of fetal nuchal translucency at 11 to 13 + 6 (median, 12) weeks of gestation. A 3D volume of the fetal face was acquired and then analyzed using the multiplanar mode. A sequence of transverse views was used to confirm the presence or absence of the nasal bones and when they were present any visible gap between them was measured. A perfectly mid-sagittal plane was then examined to determine if the nasal bone was visible or not. RESULTS In 421/450 (93.6%) cases the nasal bone was present during 2D ultrasound. Using the multiplanar mode of 3D ultrasound, in 83/421 (19.7%) fetuses a gap between the nasal bones could be demonstrated and in 36/83 (43.4%) cases the nasal bone was found to be absent in the perfect mid-sagittal view. In 29/450 (6.4%) cases the nasal bones were absent during the 2D scan. In the 3D assessment there was absence of both bones in 25/29 (86.2%) cases and absence of one of the two bones in 4/29 (13.8%) cases. Chorionic villus sampling demonstrated that the fetal karyotype was normal in 404 and abnormal in 46 cases, including 31 cases of trisomy 21. There was absence of one or both nasal bones in three (0.7%) of the chromosomally normal fetuses, in 19 (61.3%) with trisomy 21 and in seven (46.7%) with other chromosomal defects. CONCLUSIONS At 11 to 13 + 6 weeks of gestation there is a gap between the nasal bones in about 20% of fetuses, and in about 40% of these cases in the perfect mid-sagittal plane the nasal bone may erroneously be considered to be absent.