Is the lower the NT value, the better?

The nt value mainly refers to the nuchal translucency, which mainly refers to the maximum thickness from the skin to the subcutaneous soft tissue at the horizontal section of the fetal cervical spine. When scanning, it can be checked by B-ultrasound. This examination can assess whether there is a problem with Down syndrome. If the nt value exceeds the normal range, a fetal malformation examination should be performed at this time. Let’s take a look at this aspect.

Is the lower the NT value, the better?

NT (Nucleus Translucency Scan) is a method of assessing whether a fetus may have Down syndrome, which is a screening test. Unlike diagnostic tests such as chorionic villus sampling or amniocentesis, there is a definitive diagnosis, a clear karyotype. The ultrasound examination of NT depends on the doctor's diagnostic experience and the sensitivity of the ultrasound machine. It is a screening test and the results can prompt pregnant women to choose which further examination is more appropriate.

Nuchal translucency B-ultrasound scan is usually performed between 11 and 13 weeks + 6 days of pregnancy. Scanning before 11 weeks is technically difficult because the fetus is too small, and after 14 weeks, the excess fluid may be absorbed by the fetus's developing lymphatic system, making the results inaccurate. This screening is done mainly with ultrasound scans, usually of the abdomen.

However, the position of the fetus and uterus also needs to be considered. If necessary, vaginal B-ultrasound should be performed so that a clearer view can be obtained.

The nuchal translucency grows accordingly as the fetus grows. Ultrasound doctors believe that the normal thickening of the nuchal translucency of a fetus does not exceed 2.5 mm, and it is abnormal if the nuchal translucency is larger than 3 mm. The thicker the nuchal translucency, the higher the risk that the fetus will have Down syndrome. If the thickness reaches 6 mm, there is a high risk of Down syndrome and other chromosomal, genetic syndromes and heart problems.

Therefore, if a pregnant woman's NT value is detected to be outside the normal range during the NT test, further fetal abnormality screening tests should be performed, such as amniocentesis or chorionic villus sampling, to determine whether she has Down syndrome or other diseases.

How to do NT test

Transvaginal ultrasonography was performed in pregnant women at 11 to 14 weeks of gestation to measure the thickness of the fetal nuchal translucency.

1. During ultrasound examination, the crown-rump length (CRL) and biparietal diameter of the fetus are first measured to determine the ultrasound gestational age. Then the fetal structure is checked in sequence, and some important indicators such as fetal heart rate, fetal movement, the connection between the umbilical cord and the fetus, the development of the fetal limbs, and the intracranial choroid plexus are observed to exclude fetal abnormalities such as holoprosencephaly, choroid plexus cysts, umbilical hernias, and other developmental abnormalities.

2. The thickness of the nuchal translucency is measured in the sagittal section of the fetal long axis, with the fetal head and spine in a straight line during measurement. Adjust the magnification so that the fetal body occupies about 3/4 of the screen size. The measurement point is placed at the inner edge of two high-echo lines for measurement. These two high-echo lines represent the skin echo and the soft tissue echo on the spine surface respectively, and the part between them is the nuchal translucency.

3. Take a sagittal section of the fetal long axis, with the fetus in a naturally flexed position, and measure the thickest part of the subcutaneous echo-free transparent layer in the fetal neck, from the inner edge of the skin layer to the outer edge of the fascia layer. Measure three times and take the thickest value.

4. If the thickness of the fetal nuchal translucency is measured to be ≥3mm (or ≥2.5mm and the gestational age calculated based on the fetal crown-rump length is <12 weeks), the pregnant woman is recommended to undergo amniocentesis or chorionic villus sampling for fetal chromosome karyotype analysis.