What Are the Most Common Terms in First Trimester Screening Ultrasound?

What is the abbreviation for NT and NB in first trimester screening ultrasound and what is its importance?

In first trimester screening ultrasound, two parameters are of great value in ultrasound, first the size of NT and second the size of NB.

Seeing these two and their size is very important.

NB stands for nasal bone (nasal septum) and NT stands for Nuchal Translucency (size of the back of the fetus’s neck). These two parameters are key points in determining trisomy 21 and some fetal abnormalities.

The normal size of NB in anomaly ultrasound is determined according to the gestational age. Of course, NB, which stands for nasal bone, is measured in NT ultrasound in the first trimester of pregnancy and compared with the normal value at that age.

Seeing NB or NT is one of the most important things in first trimester screening ultrasound and ideally it should be seen in the image selected for NT measurement. Three lines should be seen in the nasal image. The upper line represents the skin. The lower line, which is thicker and more echogenic than the skin, is the nasal bone, and the third line, which is above the skin and continues from it, represents the tip of the fetal nose. The figure below shows the fetal nose on ultrasound.

The importance of the fetal nasal bone in fetal health

In general, in 60 to 70 percent of cases with Down syndrome (trisomy 21), 50 percent of type 18 syndrome, and 30 percent of type 13, the fetal nasal bone is absent. For this reason, the absence of the fetal nasal bone in NT ultrasound is a very important sign and requires fetal chromosomal tests.

Normal NB value in ultrasound anomaly

A value of 4.5 mm is normal for NB at about 17 weeks of pregnancy. The more important issue is that the NB is not visible, and another issue is that the NB is small (in most cases less than 2.5 mm at weeks 15-20), and also that the NB size is smaller than the 2.5 percentile is abnormal. In general, the normal size of the NB depends on the gestational age. Below is an example of the measurement of the NB on a first trimester screening ultrasound.