Posterior nuchal translucency A posterior nuchal translucency (NT) scan is a way to assess whether your baby may have Down syndrome. The fluid that accumulates under the skin at the back of your baby’s neck between 10 and 14 weeks of pregnancy can be measured with ultrasound. All babies have some fluid, however, most babies with Down syndrome have a thicker nuchal translucency (NT). This is a screening test that can only assess your baby’s risk of having Down syndrome, unlike diagnostic tests like chorionic villus biopsy or amniocentesis, which will give you a definitive diagnosis (although it also carries a small risk of miscarriage). Although a posterior nuchal translucency scan cannot tell exactly if your baby is sick, it can help you decide if diagnostic testing is needed. Timing of the test The posterior nuchal translucency scan is usually performed between 11 and 13 weeks + 6 days of pregnancy. it is technically difficult to perform the scan before 11 weeks because the baby is too small, and after 14 weeks, too much fluid may be absorbed by the baby’s developing lymphatic system. Where the test is performed In China, a posterior cervical zona pellucida scan is generally recommended for people at high risk for Down syndrome, such as older pregnant women older than 35 years old, those who have previously delivered a child with Down’s syndrome or have a family history of delivering a child with Down’s syndrome. The posterior nuchal translucency scan can usually be done at a large hospital (tertiary hospital) or a large specialized maternity hospital. Screening method This screening is done mainly by ultrasound scan. Ultrasound is usually done 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. If conditions allow, you can go to the ultrasound with your husband and share this memorable experience together. Measuring the results Your results should be known immediately. In the test result report form, you will be able to see the value of the thickness of your baby’s posterior cervical zona pellucida. If the result is abnormal, please make sure to consult your doctor. Normal condition The posterior cervical zona pellucida usually grows accordingly with the growth of the baby. In China, doctors consider the posterior cervical zona pellucida to be abnormal if it is greater than 3 mm. Critical state If the nuchal translucency is thick, it does not necessarily mean that your baby has a problem. Nine times out of ten, babies who measure between 2.5 and 3.5 mm are perfectly normal. The thicker the posterior cervical zona pellucida, the higher the risk of disease. The baby in Figure 3 has a posterior cervical zona pellucida of 6 mm and is at high risk for Down syndrome as well as other chromosomal and genetic syndromes and heart problems. Fortunately, not many babies will have such a thick nuchal translucency. Risk assessment All women are at risk of having a baby with Down syndrome, and this risk rises with age. First, the statistics will show the risk associated with the age of the mother-to-be, which is called the background risk. Second, there is the measurement derived from the ultrasound that is your own individual risk for this pregnancy. This new risk value may be higher or lower than your background risk. Response Although about 1 in 20 women will get a high-risk result, the vast majority will have a healthy baby. Even at a high risk of 1:5, there are still 4/5 babies who are not Down’s. However, once you are marked as high risk, it is likely that you will be anxious and unsure of what to do. The only way to know exactly if your baby has Down syndrome or another defect is to have a diagnostic test like a chorionic villus biopsy or amniocentesis. Making this decision can be very difficult, and you don’t have to act in haste. One of the benefits of a posterior nuchal translucency scan is that it is tested early in pregnancy, a time when it is still possible to have a chorionic villus biopsy and know the results early. If you are not sure what to do, then you can wait until 16 weeks to have an amniocentesis if you wish, or you can ask your doctor for more information. Test reliability A large study of more than 10,000 mothers-to-be found that about 75% of babies with Down’s syndrome were correctly identified by this method. When combined with blood tests, this rate increases to about 90 percent. However, in order to achieve this rate, the posterior cervical zona pellucida must be accurately measured. Ultrasonographers in China are required to take an imaging exam and be certified to perform ultrasound, and the physician who performs the posterior cervical zona pellucida scan is usually relatively fixed.