What is NT?

NT examination, also known as "back neck clear layer scanning", uses B-ultrasound examination to measure the thickest part of the subcutaneous non-echo transparent layer of the fetus neck to evaluate whether the fetus has Down syndrome. Studies have shown that fetal NT thickening in early pregnancy is related to increased risk of adverse pregnancy outcomes such as fetal chromosomal abnormalities, fetal congenital heart disease and other structural abnormalities. Therefore, NT measurement is also regarded as a key screening item in early pregnancy.

When should I do NT check?

The change in NT value is related to the gestational week, and it has strict restrictions on the examination time:

The NT test is most suitable between 11-13 weeks and 6 days of pregnancy;

The head and hip length is between 45-84mm.

As early as 11 weeks, the embryo is very small, which is not conducive to examination; after 14 weeks, excessive fluid will be absorbed by the fetal lymphatic system.

So pregnant mothers don’t miss the best time for examination!

What is the value of NT normal?

For the definition of normal values, the 99th percentile or the 95th percentile is generally used in medicine as the cleavage value. For example, 95% of the people in the population are less than 1.8 meters tall, 99% are less than 1.9 meters tall, if we use 1.8 meters as the standard, 5% are defined as "unusual height"; if we use 1.9 meters as the standard, 1% are defined as "unusual height".

For NT values, the 99th percentile is generally 3 mm, and 2.5 mm corresponds to the 95th percentile. With 2.5mm or 3.0mm as the cutting value, each prenatal diagnosis center is not completely unified. It is generally believed that when the NT value is greater than the 99th percentile, the NT value will thicken. Therefore, most diagnostic standards currently use 3.0mm as the standard. It is believed that when the NT thickness exceeds the 99th percentile, it will thicken NT, that is, 1% of pregnant women are defined as "abnormal", and then further screening is performed. In clinical practice, some pregnant women have NT measurement results between 2.5-3.0mm, which is a "gray zone". Doctors need to further confirm the 95th percentile or the 99th percentile of the NT value corresponding to the fetal head and hip diameter.

The larger the NT value, the higher the risk of chromosomal abnormality. When the NT thickness is 5.5-6.4 mm and greater than 8.5 mm, the incidence of chromosomal abnormality is 50% and 75% respectively.

Is the NT check accurate?

It needs to be discussed in accordance with individual circumstances. Some normal fetal fluids are more likely to interfereMeasurement results. If the transparent belt behind the neck is thick, it does not mean that there must be something wrong with the fetus. On the other hand, NT examinations also test the skills of B-ultrasound doctors. Data show that the accuracy of this examination is 60-70%.