Breeding a baby is a long and happy process. Only by doing every physical examination carefully can we ensure the birth of a healthy baby. Among them, early pregnancy doctors will recommend pregnant mothers to do NT examinations. Why do you have to do this check, is it important and meaningful?
One of the key examinations in early pregnancy
The fetal neck hyaline band examination, referred to as NT, refers to the liquid transparent band that appears in the posterior part of the fetus in the early pregnancy. In the early development process, the lymphatic system and venous system of the fetal neck are not yet perfect, a small amount of lymph fluid can not be completely absorbed and accumulate in the lymphatic sac or lymphatic vessels. The subcutaneous anechoic zone of the fetal neck can be seen under ultrasound. The skin is echoed between the high echo zone and the deep soft tissue hyperechoic zone.
As early as 1992, Nicolaides et al. from the United Kingdom first reported the correlation between NT thickness and fetal chromosomal disease. By measuring fetal NT thickness during early pregnancy, it can be assessed that the fetus suffers from Down syndrome ( Also known as the risk of chromosomal diseases such as congenital or Down syndrome. In the subsequent clinical practice, NT thickening and severe fetal structural abnormalities, cardiac abnormalities, genetic syndrome, infection, etc. were also found to be relevant. Since then, it has become one of the key indicators of early pregnancy ultrasound examination.
According to the “Prenatal Ultrasound Guidelines” issued by the Ultrasound Physician Branch of the Chinese Medical Doctor Association in 2012, all pregnant mothers are suitable for early pregnancy during the fetal head and hip length of 45-84 mm (equivalent to 11-13 + 6 weeks of pregnancy).
This test can be used alone as an indicator of non-invasive prenatal screening in early pregnancy and has a unique advantage in prenatal screening for twin pregnancies. In the case of reliable detection, it is recommended that B-hCG (human chorionic gonadotropin) and PAPP-A (abbreviation of pregnancy-related protein) in the blood of pregnant women during early pregnancy are the markers of Down’s syndrome (DS) screening. a) together with the serological indicators, combined with the clinical information of pregnant women, gestational age, past history and other clinical information, calculate the probability of fetal chromosomal disease. In singleton pregnancies, triple screening (NT, B-hCG, PAPP-A) in early pregnancy can detect more than 85% of the most common trisomy 21 syndrome (Down’s syndrome).
In addition to the NT index, more information will be examined during NT examinations in early pregnancy, including: number of fetuses and chorionicity: fetal heartbeat: fetal head-to-hip length (CRL), nasal bone (NB) condition: fetal appendage (placental position) , placental thickness, maximum depth of amniotic fluid, etc.).
How to read the inspection report
After the pregnant mothers complete the NT examination, they usually notice the description of “fetal NT thickness XX mm” in the report, which is an important result of NT examination in early pregnancy.
The normal range of NT increases with increasing gestational age. In general, the vast majority of babies have an NT value <2.5 mm, which is a low risk for NT screening; however, when NT ≥ 2.5 mm, the pregnant mothers need to pay attention. In general, the thicker the NT, the higher the risk of the fetus. In addition, about 80% of fetal structural malformations have occurred at 12 weeks of gestation, including bones, brain, abdominal wall, heart, spine, extremities, abdomen, bladder, etc., which are also observed in NT testing. What should I do if I have an inspection exception? Some pregnant mothers will be very flustered when they get abnormal results from NT examinations. In fact, NT thickening is not exactly equal to the fetus. At 10-14 weeks of gestation, the lymphatic and venous systems of the fetal neck will complete the developmental connection. Before this, the fetus will have a transient lymphatic drainage disorder, most of which is simple NT thickening (temporary thickening), after which Naturally subsided, so pregnant mothers should not be blindly nervous when receiving abnormal results, the doctor will help you to investigate possible risks. According to the existing research reports, the examination suggests that the NT thickening of the fetus may have the following problems: abnormal chromosome number, congenital heart defect, some other structural malformations, genetic syndrome, twin transfusion syndrome, infection, etc. Chromosomal abnormalities Among all the factors that cause NT thickening, chromosomal abnormalities are the most common cause. In the 21 trisomy syndrome (Down's syndrome, congenital type), about 75% of NT will be thickened: 18 trisomy (main bone, genitourinary system, heart and other deformities), 13 trisomy (head appearing Fetuses with defects and deformities of the brain and heart) and aneuploidy of the sex chromosomes (resulting in infertility, miscarriage, embryonic cessation and fetal defects), the NT index is also significantly thicker than the normal fetus. Therefore, when NT thickening occurs in the fetus, chromosomal abnormalities must first be excluded. Congenital heart defects In congenital heart defects and other structural abnormalities, congenital heart defects and other structural abnormalities are also part of the inability to ignore. Congenital heart defects are the most common birth defects at present. In general, most fetal heart-related malformations are not easily detected during pregnancy. Studies have found that 55% of heart-deficient fetuses have NT thickening, so when NT thickening exists, fetal heart ultrasound testing is needed to check for related defects. Other structural malformations In many studies, there have been reports of NT thickening associated with fetal structural abnormalities, and some structural abnormalities may be discovered during NT examination. Some of the defects are fatal, requiring timely therapeutic intervention, and part of the fetus. There will be significant effects that may cause birth defects. When fetal structural malformation is found, it is recommended to use genetic techniques such as karyotype, CMA (chromosome chip analysis, chromosomal microarray anal-ysiS), or even external explicit sequencing (WES) with sufficient informed consent under the guidance and guidance of a doctor. Troubleshoot possible genetic causes. NT measurement of twin pregnancy in twin pregnancy is also of value in predicting twin-transfusion syndrome. When the difference in NT thickness of the twins is ≥0.6 mm, the specificity of the twin-transfusion syndrome is predicted to be as high as 92% and the sensitivity is up to 50%. Other NT thickening may also be associated with infection during pregnancy (eg, parvovirus B19 infection), fetal growth restriction (FGR), fetal anemia, hypoproteinemia, and the like. The NT examination is an important ultrasound screening program for all pregnant women during the first trimester (11-13 + 6 weeks gestation). NT thickening (≥2.5 mm) is only a hint of risk and does not mean fetal abnormalities. It is recommended to perform further testing under the guidance of a specialist (genetic counselor, obstetric specialist, fetal medical specialist).