What is a posterior cervical zona pellucida scan? This is the fluid that accumulates under the skin at the back of your baby’s neck between 10 and 14 weeks of pregnancy and can be measured with ultrasound. All babies have some fluid, however, most babies with Down syndrome have a thicker nuchal translucency band (NT). A posterior nuchal translucency scan is one way to assess whether your baby may have Down syndrome; it is a screening test. Unlike screening, which only assesses your baby’s risk of having Down syndrome, diagnostic tests like chorionic villus biopsy or amniocentesis will give you a definitive diagnosis (although they also carry a small risk of miscarriage). In China, a posterior nuchal translucency scan is generally recommended for people at high risk for Down syndrome, such as older women older than 35 years old, who have previously delivered a child with Down’s syndrome or who have a family history of delivering a child with Down’s syndrome. A posterior nuchal translucency scan can usually be done in a large hospital (tertiary hospital) or in a large specialized maternity hospital. Although the posterior nuchal translucency scan cannot tell exactly if your baby is sick, it can help you decide if diagnostic testing is needed. What is the NT test? A fetus with Down’s syndrome will have subcutaneous fluid retention and therefore thicker skin at the back of the neck. If NT detects fetal skin thickness at the nape of the neck that exceeds the standard value, it may be associated with fetal karyotype abnormalities and other structural abnormalities, and the thicker the NT, the greater the probability of fetal structural abnormalities and chromosomal abnormalities. In addition to detecting the risk of chromosomal abnormalities, thickening of the nuchal translucency is also associated with congenital heart disease. If the NT test result is out of the standard value range, pregnant women are recommended to undergo subsequent screening tests to further confirm the risk of fetal abnormalities so that early countermeasures can be taken. Need for NT test In China, doctors generally recommend a posterior nuchal translucency scan for people at high risk for Down syndrome, such as older pregnant women older than 35 years old, those who have previously delivered a Down’s child or have a family history of delivering a Down’s child. Although a posterior nuchal translucency scan cannot determine with certainty whether the fetus has the disease, it can help the pregnant woman decide whether further diagnostic testing is needed. Unlike diagnostic tests such as NT and chorionic villus biopsy or amniocentesis, which provide a definitive diagnosis, the NT test can only indicate risk. The accuracy of the posterior nuchal translucency test is determined by many factors, including the level of the ultrasonographer and the accuracy of the scanner used, and therefore the results are not guaranteed to be 100% accurate; the NT test only provides a reference for the need for further prenatal diagnosis and cannot be used to determine whether the fetus is abnormal or not. To determine whether the fetus has chromosomal abnormalities, further chromosomal analysis must be performed by chorionic villus sampling or amniocentesis. When is a posterior cervical zona pellucida scan done? A posterior nuchal translucency scan is usually performed between 11 and 14 weeks of gestation. it is technically difficult to perform an NT test before 11 weeks because the fetus is still too small at this time. And after 14 weeks, the fluid under the fetal skin may be absorbed by the developing lymphatic system, making it impossible to produce definitive test data. Therefore, pregnant women need to go to the hospital for NT screening within 11-14 weeks of pregnancy. The posterior nuchal translucency scan can usually be done at a tertiary care hospital or a large specialist maternity hospital. How is a posterior nuchal translucency scan done? This screening is mainly done by ultrasound scan, usually an ultrasound on your belly, but it also depends on the position of the baby and the uterus, and if necessary, by vaginal ultrasound, so that you can see more clearly. A vaginal ultrasound is not risky for you or your baby and should not cause much discomfort. To accurately determine your pregnancy, the ultrasonographer will measure the baby’s head-rump length and the width of the zona pellucida at the back of the neck. Under the ultrasound, your baby’s skin looks like a white line, while the fluid under the skin looks black. Your baby looks clear at this stage, and you can also see his head, spine, limbs, hands and feet. Although some major abnormalities may be able to be ruled out in this ultrasound, it is still recommended that you have a detailed ultrasound at 20 weeks. Standard NT test values The posterior cervical zona pellucida usually grows accordingly with the growth of the baby. In China, doctors consider a posterior nuchal translucency band of no more than 3 mm to be normal. Interpretation of NT test data Reasons for high NT value NT value exceeds the normal range may be related to the following: 1. During normal embryonic development, the jugular lymphatic vessels and the jugular venous sinus are connected at around 11-14 weeks. Before the connection, a small amount of lymphatic fluid accumulates in the neck to form a temporary NT thickening. It should subside after 14 weeks in normal fetus. If the cervical lymphatic vessels and jugular venous sinus are delayed to connect, thus there is obvious obstruction of cervical lymphatic reflux and excessive accumulation of lymphatic fluid in the neck, NT thickening is obvious, and even develops into lymphatic hydrocystic tumor by the middle of pregnancy. 2. Fetuses with normal karyotype and congenital heart malformation often have thickened NT, which may be related to heart failure. In heart failure, venous return is impaired, resulting in increased jugular venous pressure, obstruction of lymphatic fluid return in the lymphatic vessels, and excessive accumulation of lymphatic fluid in the neck, resulting in NT thickening. 3.The extracellular hyaline matrix of the skin of the neck increases in Down syndrome fetuses, and extracellular fluid is adsorbed in a large amount in the interval of the hyaline matrix, causing spongy changes in the skin of the neck and forming NT thickening. 4, congenital diaphragmatic hernia, due to herniation of abdominal contents into the thoracic cavity to increase the pressure in the thoracic cavity, occupying lesions in one lung, due to the increase in the volume of that side of the lung, resulting in mediastinal displacement of venous return obstruction, lethal skeletal malformation of the thoracic cavity extremely narrow intra-thoracic pressure, etc. can cause venous return obstruction and venous congestion. When the venous blood returns to the head of the neck, NT appears thickened. The NT test can determine the sex of the fetus, but the focus of the NT test is on the thickness of the posterior nuchal translucency, while the doctor needs to use an ultrasound device to look at the genitals of the fetus to determine the sex of the fetus, so the NT test alone cannot determine the sex of the fetus. NT test precautions 1, NT (posterior nuchal translucency scan) test is an ultrasound test, no blood test is needed, eating and drinking will not affect the test results, so fasting is not needed before the test. 2.Since the fetus is too small to observe the posterior cervical zona pellucida before 11 weeks of pregnancy, and after 14 weeks of pregnancy, due to the gradual development of the fetus, the excess body fluid out of the cervical zona pellucida may be absorbed, which will affect the test results, so it is better for pregnant women to go for NT within 11-14 weeks of pregnancy to avoid inaccurate test results. 3. It is best to make an appointment for NT in advance, usually before 11 weeks of pregnancy, so that you can start making an appointment with the hospital for scheduling. Do not make an appointment after 13 weeks of pregnancy to avoid long queues. Doing NT beyond the pregnancy week will affect the accuracy of the test results. NT test response Once a pregnant woman is detected with a high NT, the only way to know exactly if the fetus has Down syndrome or other defects is to have a diagnostic test with a chorionic villus biopsy or amniocentesis. The timing of the posterior nuchal translucency scan is early in pregnancy, a time when a pregnant woman can still have a timely chorionic villus biopsy and learn the results early if an abnormality is detected. If a pregnant woman does not know what to do, she can wait until 16 weeks of pregnancy to have an amniocentesis, or she can ask her doctor directly for more information and advice.