ad. 11. What does the absence of a nasal bone in the first trimester mean?
“The absence of a nasal bone” is not a very accurate term describing a state, in which the fetus has a delayed ossification of the nasal bone. A lot of patients even believe that the nose of their baby had not developed. Nothing of the kind! The nose exists in such cases. We would rather encourage to use the terms: delayed or correct ossification of the nasal bones.
In clinical understanding the term “delayed ossification of the nasal bones” between 11 and 14 weeks occurs frequently, especially if the examination is performed at 11 weeks. In such cases, when a delayed ossification is an isolated feature, it is recommended to wait and undergo a check-up examination after one week time. Usually, the calcification of the nasal bones improves and we can calm you and ourselves. However, if the ossification is still delayed, and the risk of genetic syndromes is increased, it is recommended to undergo a conclusive examination, i.e. chorionic villus sampling or amniocentesis.
In the first trimester we do not measure the length of the nasal bone, we evaluate the fetus for the presence or absence of the nasal bone and assess the calcification of one of the two nasal bones.
In our records, a delayed ossification of the nasal bones in the group of healthy fetuses we have observed so far in 2.6% of cases, in the group of the Down syndrome in 40.5%, in the group of the Edwards’ syndrome in 36.4%, in the group of the Patau syndrome in 50%, and in the group of the Turner’s syndrome in 44.4%.
A weak ossification of the nasal bones in the second trimester between 15 and 22 weeks is also considered an indication to perform amniocentesis. The difference in the assessment of the ossification in the second trimester consists in the measurement of the length of the bones (a correct value is 2.5mm) and the assessment of symmetry of both of them.