Doctors and parents all want the best for a pregnancy. Nowadays, medicine is more focused on identifying potential problems early and taking action to prevent them, rather than just diagnosing, and treating issues. This same principle applies to checking on the health of the baby in the womb. The NT-NB scan, which is typically done around the third month of pregnancy, plays a crucial role in this.
One thing the NT-NB scan can help with is checking for chromosomal abnormalities in the baby. Chromosomes are tiny structures in our cells that carry genetic information, and we usually have 46 of them – 23 from our mother and 23 from our father. Chromosomes determine how our bodies develop and work. Problems with chromosomes can cause lasting changes in how the body functions, and these changes can’t be fixed. Some common chromosome problems include Down syndrome, Edwards syndrome, and Patau syndrome. Down syndrome is the most common, affecting about 1 in every 600 babies born. Children with Down syndrome have an extra chromosome, which means they have 47 instead of the usual 46. This extra chromosome can lead to problems with different parts of the body and can affect how the brain develops, which can result in learning difficulties.
To check the number of chromosomes in the baby, doctors can take a sample of the fluid around the baby and examine the cells in that fluid. This procedure is called amniocentesis, but it comes with a small risk of miscarriage (about 1 in 1000), so it’s not suitable for everyone. That’s why we use “screening” tests for chromosome problems. Screening is a bit like taking a first look to see if there’s a risk of a problem, but it can’t give a definite answer.
When it comes to screening for chromosome problems, especially Down syndrome, it usually happens between the 11th and 13th week of pregnancy. It involves two things: the NT-NB scan and a blood test for the mother. During the scan, the doctor checks for certain signs, like the thickness of the fluid at the back of the baby’s neck (NT) and whether there’s a tiny bone in the baby’s nose (NB). They also look at the blood flow in the baby’s heart and a blood vessel. Most babies with Down syndrome have thicker neck fluid or no bone in their nose, and sometimes their heart and blood vessel don’t look quite right. Based on these signs, doctors estimate the chances of the baby having Down syndrome. However, some healthy babies can also show these signs, and some babies with Down syndrome may not show any of them and look completely normal on the scan.
To improve the chances of finding babies with Down syndrome, doctors also do a blood test called the “double marker test.” This test looks at two chemicals in the mother’s blood: Beta HCG (Human chorionic Gonadotropin) and PAPPA (Pregnancy Associated Plasma Protein A). If the baby has Down syndrome, these chemicals might be at different levels. The blood test and the scan results are combined to give a “combined risk,” which tells us how likely it is that the baby has Down syndrome.
The final risk is divided into “high risk”, “intermediate risk” and “low risk.” If the risk is more than 1 in 100, like 1 in 60, it’s considered high. If it’s less than 1 in 1000, like 1 in 3000, it’s considered low. If the risk is between 1:100 to 1:1000, like 1:800, it is considered intermediate risk. For women with low-risk results, the chance of the baby having Down syndrome is very low, and they typically don’t need any more tests. But for women with high-risk results, the chance of the baby having Down syndrome is higher. In such cases, they might be offered further tests, such as amniocentesis, to be more certain.
For women who are in the middle when it comes to risk (intermediate risk), there’s another test called NIPT, which stands for Non-invasive prenatal testing. It’s a blood test for the expectant mother, and it’s a relatively new way to check for common chromosome problems in the baby during pregnancy.
In this test, the doctor takes a small sample of the mother’s blood. They use advanced DNA technology to look at the baby’s genetic material in that blood sample. This is possible because the genetic material, which is called DNA, from the placenta (a part of the pregnancy) ends up in the mother’s blood. The placenta has the same kind of genetic material as the baby, so by testing the placental DNA in the mother’s blood, we can get an idea of the baby’s genetic material. NIPT is also a screening test, which means it helps us estimate the risk of a problem. If the risk is low, usually no more tests are needed. But if the risk is high, the doctor might recommend a more definite test which is the amniocentesis, to be sure.
In simple terms, the NT-NB scan and related blood tests help doctors figure out the chances of a baby having Down syndrome or other chromosomal issues. It’s important because knowing this early on can help parents and doctors make informed decisions about the pregnancy and prepare for any extra care the baby might need.
