If you recently found out you’re pregnant, you’ll have several doctor appointments and screenings from now until the birth of your baby. Prenatal screenings can identify problems with your health, such as anemia or gestational diabetes. Screenings also can monitor your baby-to-be’s health, and help identify chromosomal abnormalities.
Pregnancy screenings take place during the first, second, and third trimesters. The first trimester screening is a type of prenatal testing that provides your doctor with early information about your baby’s health — namely your baby’s risk for chromosome abnormalities.
A nuchal translucency (NT) scan screens your baby for these abnormalities. This test is typically scheduled between weeks 11 and 13 of pregnancy.
What’s the purpose of an NT scan?
An NT scan is a common screening test that occurs during the first trimester of pregnancy. This test measures the size of the clear tissue, called the nuchal translucency, at the back of your baby's neck.
It’s not unusual for a fetus to have fluid or clear space at the back of their neck. But too much clear space can indicate Down syndrome, or might show another chromosome abnormality like Patau syndrome or Edwards syndrome.
Our body cells have many parts, including a nucleus. The nucleus holds our genetic material. In most cases, the nucleus has 23 pairs of chromosomes, which are equally inherited from both parents.
Individuals born with Down syndrome have an extra copy of chromosome 21. Down syndrome, which can’t be cured, causes developmental delays and distinct physical characteristics.
- a small stature
- eyes with an upward slant
- low muscle tone
This condition affects 1 in every 700 babies born in the United States. It's one of the most common genetic conditions.
Patau syndrome and Edwards syndrome are rare and often fatal chromosome abnormalities. Unfortunately, most babies born with these abnormalities die within the first year of life.
When is an NT scan scheduled during pregnancy?
The clear space in the back of a developing baby’s neck can disappear by week 15, so an NT scan should be completed in the first trimester.
This test can also include blood work to measure your levels of plasma protein and human chorionic gonadotropin (HCG), a maternal hormone. Abnormal levels of either may indicate a chromosome problem.
How does an NT scan work?
During the screening, your doctor or a technician will take an abdominal ultrasound. You could alternatively have a transvaginal test, where an ultrasound probe is inserted through your vagina.
An ultrasound uses high frequency sound waves to create an image from inside your body. From this image, your doctor or technician measures the translucency, or clear space, at the back of your baby’s neck. They can then enter your age or date of birth in a computer program to calculate the risk of your baby having an abnormality.
An NT scan cannot diagnose Down syndrome or any other chromosome abnormality. The test only predicts the risk. Talk to your doctor about available blood tests. They also can help assess your baby’s risk.
As with any prediction, the accuracy rate varies. If you combine an NT scan with blood testing, the screening is about 85 percent accurate for predicting the risk of Down syndrome. If you don’t combine blood testing with the scan, the accuracy rate drops to 75 percent.
How to prepare for your test
No special preparation is required for an NT scan. In most cases, testing is completed in about 30 minutes. During the scan, you’ll lie down on an exam table as the technician moves an ultrasound wand over your stomach.
The ultrasound pictures may be easier to read if you have a full bladder, so your doctor may recommend drinking water about one hour before your appointment. The ultrasound tech needs access to your lower abdomen, so make sure you wear comfortable clothing that makes it accessible.
Results from the scan may be available on the same day of testing, and your doctor may discuss the findings with you before you leave. It's important to remember that receiving an abnormal result from an NT scan doesn’t necessarily mean that your baby has a chromosome problem. Similarly, normal test results can’t guarantee that your baby won’t be born with Down syndrome.
This test isn't perfect. There’s a 5 percent false-positive rate. In other words, 5 percent of women tested receive positive results, but the baby is fine. After a positive result, your doctor may suggest another blood test called prenatal cell-free DNA screening. This test examines fetal DNA in your bloodstream to assess your baby's risk for Down syndrome and other chromosome abnormalities.
Screening vs. diagnostic testing
It can be frightening to receive inconclusive or positive results from an NT scan. Keep in mind that an NT scan can only predict your baby’s risk: It doesn't offer a definitive answer about chromosomal abnormalities. An NT scan is a screening test, not a diagnostic test.
There are differences between screening and diagnostic testing. The purpose of a screening test is to identify risk factors for a particular disease or condition. Diagnostic testing, on the other hand, confirms the presence of a disease or condition.
How to diagnose an abnormality
To diagnosis a chromosome abnormality, ask your doctor about diagnostic testing. Options include an amniocentesis, which is when a needle is inserted through your stomach into the amniotic sac to retrieve a fluid sample. Amniotic fluid contains cells that provide genetic information about your baby.
Another option is chorionic villus sampling. A sample of your placental tissue is removed and tested for chromosome abnormalities and genetic problems. There’s a small risk of miscarriage with both tests.
An NT scan is a safe, noninvasive test that doesn’t cause any harm to you or your baby. Keep in mind that this first trimester screening is recommended, but it’s optional. Some women skip this particular test because they don’t want to know their risk. Talk to your doctor if you experience anxiety, or are worried about how the results might affect you.