What causes high NT values?

NT (nuchal translucency) refers to the maximum thickness between the skin and subcutaneous soft tissue in the sagittal cross-section of the fetal cervical vertebrae. The nuchal translucency scanner uses B-ultrasound to examine the thickness of the skin on the back of the fetus' neck to assess whether the fetus is likely to have Down syndrome. So, what is the reason for the higher NT value?

What does NT check?

Babies with Down syndrome will have water retention in the subcutaneous tissue, so the skin on the back of the neck will be thicker. If the NT test shows that the thickness of the fetal neck skin exceeds the standard value, it may be related to abnormalities in the fetal chromosomal karyotype and other anatomical abnormalities. The thicker the NT, the greater the chance of fetal anatomical abnormalities and chromosomal abnormalities.

In addition to detecting the risk of chromosomal abnormalities, thickening of the fetal nuchal translucency is also related to congenital heart disease.

If the NT test result exceeds the index range, pregnant women are advised to conduct subsequent fetal abnormality screening to further determine the risk of fetal abnormalities so that countermeasures can be taken as soon as possible.

NT value is within normal range

The nuchal translucency generally increases with the growth and development of the fetus, and will gradually disappear after 14 weeks of pregnancy. In our country, doctors consider a nuchal translucency greater than 3mm to be abnormal. Due to different individual conditions, some normal fetuses have more blood, which can easily affect the accurate measurement results. If the nuchal translucency is thick, it does not necessarily mean that there is a problem with the fetus.

If the NT value is close to 3mm, pregnant women can consult a doctor to decide whether to conduct further tests to determine the risk of disease. Generally speaking, the thicker the nuchal translucency, the higher the risk of the fetus discovering abnormalities. If the thickness has reached 6mm, then there is a high risk of Down syndrome and other sex chromosomes, genetic syndromes and heart problems. At this time, it is recommended to conduct a subsequent risk screening to identify the condition as soon as possible and take preventive measures.

What is the reason for a high NT value?

NT value exceeds the normal range, which may be related to the following aspects:

1. During normal embryonic development, the cervical lymph vessels and jugular sinus are interconnected at around 11-14 weeks. Before communication, a small amount of tissue fluid accumulates in the neck, causing transient NT thickening. Normally, the fetus should disappear after 14 weeks. If the communication between the cervical lymph vessels and the cervical venous sinuses is delayed, resulting in obvious cervical lymphatic drainage obstruction, excessive accumulation of tissue fluid in the neck, and significant thickening of the NT, it may even develop into lymphatic cystic hygroma in the middle of pregnancy.

2. Fetuses with normal chromosome karyotype but congenital heart malformations often have thickened NT, which may be related to heart failure. When heart failure occurs, venous flow is obstructed, causing jugular venous pressure to rise. The flow of tissue fluid back into the lymphatic vessels is blocked, and excessive tissue fluid accumulates in the neck, resulting in NT thickening.

3. The transparent matrix outside the cells of the neck skin of fetuses with Down syndrome increases, and the extracellular fluid is absorbed in large quantities within the space of the transparent matrix, causing honeycomb changes in the neck skin and thickening of the NT.

4. Congenital diaphragmatic hernia: due to the herniation of abdominal contents into the chest cavity, the working pressure in the chest cavity increases. Space-occupying space in one lung causes the volume of the lung on that side to increase, leading to the movement of the mediastinum and the obstruction of venous return. Fatal skeletal deformity, extremely narrow chest cavity and increased working pressure in the chest cavity can all cause obstruction of venous return and venous hematoma. When venous blood flows back into the neck and head, the NT becomes thicker.