NT examination is mainly used to diagnose chromosomal diseases in the early stages of pregnancy and discover fetal abnormalities caused by various reasons, severe heart defects, intrauterine fetal death, and neonatal death.
What is NT?
NT is the transparent layer of the neck, which refers to the thickness of the fluid accumulation in the subcutaneous tissue fluid at the back of the fetus’s neck. It is reflected on the ultrasound image, which is the largest non-echo layer between the back of the neck skin and the subcutaneous soft tissue measured on the horizontal sagittal section of the fetal cervical spine thickness.
What is the difference between NT examination and Down's screening?
NT examination is generally carried out at 11-13 weeks + 6 days of gestation. The mid-stage of Down’s screening is at 15-18 weeks of gestation. The time is different, and the screening method is different. Sometimes the two items needs to be combined and interpreted. Down’s screening is not equivalent to NT screening. Both NT screening and Down’s screening are used to know about the health of the fetus.
When is the most appropriate time?
The zona pellucida scan is usually performed at 11-13+6 weeks of pregnancy. NT examination before 11 weeks is technically difficult to achieve because the fetus is still too small at this time. After 14 weeks, the water under the skin of the fetus may be absorbed by the developing lymphatic system, which makes it impossible to obtain exact test data. Therefore, pregnant women need to go to the hospital for NT examination within 11-13+6 weeks of pregnancy.
The following conditions must be met at the same time to measure this value
1. The fetal head and hip length is between 45-84mm.
2. Take the mid-sagittal section of the neck in the natural posture.
3. Enlarge the image of the head and upper chest as much as possible.
4. Measure the thickness in the transparent zone with the widest distance.
5. When measuring the thickness value, it should be measured at least three times, and the maximum value of the measurement should be recorded.
How to judge the examination value?
The posterior cervical zona pellucida usually increases with the gestational age. With the development of the fetal lymphatic system and venous system, the posterior cervical zona pellucida gradually disappears after 14 weeks. NT thickness exceeding the 95th percentile (equivalent to 2.5mm) can be considered as NT thickening; NT thickness exceeding 3.0mm is recognized as NT thickening.
What if the NT value is abnormal?
The zona pellucida at the back of the neck usually grows with the growth of the fetus, and will gradually disappear after 14 weeks of gestation. In China, doctors believe that the zona pellucida behind the neck is larger than 3mm as abnormal. Due to different individual conditions, some normal fetuses have more body fluids, which may easily affect the measurement results, but it does not mean that there must be a problem with the fetus. If the NT measurement value is close to 3mm, pregnant women can consult their doctors to decide whether to conduct further examinations to clarify the risk of disease. Generally speaking, the thicker the zona pellucida behind the neck, the higher the risk of abnormalities in the fetus. If the thickness has reached 6mm, there is a high risk of Down’s syndrome and other chromosomes, genetic syndromes and heart problems. At this time, it is recommended to conduct follow-up risk investigations (such as chromosome examination) to identify the condition as early as possible and make countermeasures. .
What are the precautions for NT examination?
1. NT (Zona pellucida scan behind the neck) examination is a kind of ultrasound examination. No blood test is required. Eating and drinking will not affect the examination results. Therefore, there is no need to fast before the examination.
2. NT (post-neck zona pellucida scan) examination takes the mid-sagittal section of the fetus to be measured. If the fetus’s position is not satisfied, multiple measurements are required to avoid inaccurate examination results.
3. The accuracy of the zona pellucida examination at the back of the neck is determined by many factors, including the level of the sonographer, the accuracy of the instrument used, the posture of the fetus in the uterus, and the degree of obesity of the pregnant woman. Therefore, the examination results cannot be guaranteed to be 100% accurate. NT examination is different from diagnostic tests such as chorionic villus biopsy or amniocentesis. NT examination can only indicate risks and cannot provide a definitive diagnosis.
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