- Presence of nasal bone can be observed on ultrasound as early as 11-12 weeks of gestation. It is visualized in the mid-sagittal section of the fetal face as an hyperechogenic line parallel to the nasal skin;
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- Presence of nasal bone in 12 weeks of gestation
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- Presence of nasal bone in 2nd trimester
- The presence of the well-ossified nasal bone is a reassuring feature as it is associated with a drop in risk of fetal chromosomal anomalies;
- Absence of (or hypoplastic) nasal bone is found in 0.5-1% of normal fetuses;
- It is found more commonly among fetuses with chromosomal anomalies:
- (i) Downs syndrome: 70%
- (ii) Edwards syndrome: 55%
- (iii) Patau syndrome: 40%
- (iv) Turners syndrome: 9%
- With the absence of nasal bone, it is important to have an early fetal morphology scan to look for any additional ultrasound markers for chromosomal abnormalities;
- Isolated absence of nasal bone is associated with a much lower percentage of chromosomal anomalies (0-4.5%) compared with those with the presence of other ultrasound markers/ abnormalities (75-88%);
- With the absence of nasal bone and the presence of other ultrasound markers/ abnormalities, invasive prenatal diagnosis such as chorionic villous sampling or amniocentesis is advised;
- For isolated absence of nasal bone, one should be counselled regarding the limitation of morphology scan, the pros and limitation of NIPT test and the pros and risks of invasive prenatal diagnosis.
