If you’re pregnant, your doctor might have mentioned an NT scan or first-trimester screening. An NT scan is a simple test that shows your baby’s risk for some genetic conditions.
What Is an NT Scan for Down Syndrome?
An NT scan, or nuchal translucency scan, is a non-invasive ultrasound screening for Down syndrome and other genetic conditions during pregnancy. It’s usually done between weeks 11 and 14 of pregnancy.
Some people also have blood tests done at the same time that look for certain hormones and proteins, including:
- Free beta-human chorionic gonadotropin, or b-hCG
- Pregnancy-associated plasma protein-A, or PAPP-A
- Alpha-fetoprotein, or AFP
All pregnant people have these hormones and proteins, but the levels can be higher or lower than average when the baby has Down syndrome or other disorders. These screenings together are called the combined or integrated first-trimester screening.
What Does an NT Scan Check For?
All unborn babies have a skin fold, called a nuchal fold, at the back of the neck that holds fluid. Babies with certain genetic conditions have more fluid than normal, and the nuchal fold is thicker.
An NT scan measures this fold to check for the risk of genetic conditions like:
Down syndrome. Down syndrome, or trisomy 21, is a genetic disorder where babies have an extra 21st chromosome. Chromosomes are packages of genes that tell the body how to grow. An extra chromosome changes how the brain and body develop and can cause intellectual and physical disability, birth defects, and health problems.
Trisomy 13 and 18.Trisomy 13 and trisomy 18 happen when babies have an extra copy of either chromosome 13 or chromosome 18, respectively. These genetic disorders cause serious birth defects and intellectual disability, often leading to early death.
Turner syndrome. A baby with Turner syndrome is missing all of or part of the X chromosome. This only affects babies with X chromosomes and can cause growth problems and heart and ovary defects at birth.
Congenital heart disease. Babies with congenital heart disease are born with heart defects that change how blood flows through the heart. Some defects are life-threatening, and some don’t cause any problems.
An NT scan doesn’t diagnose or confirm any of these disorders, though. Instead, it tells you whether your baby has a higher risk of having one of these conditions.
Since an NT scan is an ultrasound, your doctor might also check for other things, including:
- How your baby is growing
- How many babies you’re carrying
- Whether multiple babies share a placenta
- Exactly how far along you are in your pregnancy
What Happens at a Nuchal Translucency Scan?
An NT scan is a routine ultrasound. You need to have a full bladder, so the ultrasound technician will ask you to drink 2 to 3 glasses of water an hour before the scan. You can’t urinate before the test, so you might feel full and uncomfortable.
During the test, you'll lay down on a table and the technician will put gel on your lower belly. They'll move a wand across your abdomen and take some pictures. Once they have the pictures, your scan will be done, and you can go back to your regular activities. A radiologist will read the scans and send the results to your doctor.
What Are the Risks of an NT Scan?
There are no known risks for an NT scan. Ultrasounds are safe and non-invasive, and you can’t feel the ultrasound waves. You might feel some pressure as the technician moves the wand around your belly. This is normal and helps get the best view of your baby.
Is an NT Scan Necessary?
NT scans are optional. You have the option of getting this test early in your pregnancy, which will give you results early on. Early results can help you plan for the possibility of caring for a child with disabilities.
You can also decline this test. If you think it would cause too much stress, or if it won’t change how you manage your pregnancy or plan for your child, you might decide against it.
What Do Your NT Scan Results Mean?
The skin fold grows as unborn babies grow, so the test measures the thickness of the skin fold and compares it to babies of the same age in the womb. If the measurement is higher than the average measurement, then the risk for some genetic conditions is higher.
What is the NT scan normal range? At 11 weeks, the skin fold measures up to 2 mm. At 13 weeks and 6 days, it can be up to 2.8 mm. If your baby’s measurements are within this range, they have a low chance of having Down syndrome or other genetic disorders.
What is an abnormal NT scan measurement? An abnormal measurement is when the skin fold measure is larger than the normal range of up to 2 mm at 11 weeks or 2.8 mm at 13 weeks 6 days.
Your doctor will consider the measurements along with your age and your blood tests. While testing only shows the chances of having a condition, an NT scan with bloodwork can correctly predict genetic conditions in 85% of cases. About 5% of test results are false positives, which means they incorrectly suggest a higher chance.
What Happens if an NT Scan Is Abnormal?
If your NT scan has abnormal measurements, your doctor might suggest more testing. These tests can include:
- Chorionic villus sampling, where they test a tiny piece of the placenta
- Amniocentesis, where they test a small amount of amniotic fluid from your uterus
- Prenatal cell-free DNA (cfDNA) screening, where they take some of your blood and look at your baby’s DNA
These tests do have a risk of causing pregnancy loss, so your doctor will go over the risks and benefits for your situation. If you decide to have the test, your doctor will discuss your results with you and help you understand what they mean.
An NT scan is a safe and routine — but optional — ultrasound scan that can help predict the chances of your baby having Down syndrome. It’s sometimes done along with bloodwork in your first trimester.