What is an NT Scan (nuchal translucency scan)?

A nuchal translucency scan is one of the four scans which are done in a prenatal routine. This scan is done during 11-13.6 weeks of your pregnancy. There is a reason for this – 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 between 11-13.6 weeks of gestational age. Rather than the weeks of gestation, it is the size of the baby that helps determine when to perform the scan. The NT scan would ideally be performed when your baby measures between 45 mm and 84 mm. This ultrasound will measure your baby’s nuchal translucency (a fluid-filled space at the back of your baby’s neck) which is measured as NT in millimeters. The fluid is present in all fetuses; however, an increased collection may be a sign of chromosomal abnormalities present in conditions such as Down’s syndrome. It is called a ‘translucency’ because on an ultrasound this appears as a black space beneath the fetal skin.

Why NT is measured?

Now you may wonder why this NT is measured. So increased thickness of the nuchal translucency might indicate a chromosomal abnormality, but it doesn’t tell you that your baby definitely has, or doesn’t have, an abnormality. The results will tell you if your baby is at high risk or low risk of chromosomal abnormality in comparison to the general population. When you receive the results, it is important to understand what a normal NT measurement is. For a baby that is between 45 mm and 84 mm in size, an NT of less than 3.5mm is considered normal. An NT less than 1.3 mm is considered to be low-chance and an NT of 6 is considered a high chance for Down’s syndrome and other potential chromosomal abnormalities. This is a reference range. Your doctor will be the best person to comment on your reports. Studies have shown that an increased NT is associated with Trisomy 21 (Down’s Syndrome) and other chromosomal conditions such as Trisomy 18 (Edward’s Syndrome) or Trisomy 13 (Patau’s Syndrome), Genetic syndromes, Congenital heart abnormalities, Increased risk of miscarriages.

How is NT Scan done?

Are you eager to know how it’s done? Let me share that in detail – You’ll be referred by your doctor for your NT scan, which will normally take place at a hospital and be carried out by a sonographer. The ultrasound pictures are easier to read if you have a full bladder, so you need to drink lots of water about an hour before your appointment (i know it’s a tough job!).

The ultrasound scan is usually done abdominally so it is preferable that you wear comfortable clothing that makes your lower abdomen accessible..The scan usually takes about half an hour. You’ll be asked to lie down on your back, the sonographer will then apply gel across your lower abdomen and move a handheld device called a transducer across the gel. The scan shouldn’t be painful but you may feel some pressure as the sonographer presses on your abdomen to get a clear picture. Occasionally it can be difficult for the sonographer to get a clear scan from an abdominal ultrasound, in which case a vaginal ultrasound will be offered. This is when a small transducer will be inserted into the vagina to get a closer look at the fetus and clearer images. This method can be a little uncomfortable but is safe for the baby. The reports are given on the same day. The doctor may discuss the findings with you before you leave. You will be referred to a clinical laboratory where a blood sample will be drawn to go for a double marker test which measures the levels of certain biochemical markers in your blood like Beta HCG, PAPP-A. The results of the NT Scan and the double marker test will be taken into account for a final report.

Other benefits of NT scan.

Pre-eclampsia screening

Preeclampsia is a complication of pregnancy that causes high blood pressure and other complications in pregnant women. The fetus can also be affected, the most common risks for the baby are growth restriction and prematurity. Severe cases can result in intrauterine fetal death. The risk of developing preeclampsia is calculated by using your past history and family history, along with measurements of the blood flow in the uterine arteries, obtained at the ultrasound examination. Your blood pressure will also need to be measured to contribute to the calculations.

Cardiovascular development

The structures of the fetal heart like anatomic structure (size, rate), hemodynamic development through analysis of the waveforms and flow patterns (inflow and outflow waveforms of the diastolic filling and the systolic ejection) on the doppler scan are evaluated. Chromosomally normal first trimester fetuses with an increased nuchal translucency measurement have an elevated risk of congenital heart defect (CHD). so there is an increased demand for imaging the fetal heart during an NT scan. Major heart defects can be found through this scan.

Number of fetuses

If you missed the early pregnancy scan and this is the first scan of your pregnancy then this will give the number of fetuses whether single, twins or multiple pregnancies.

Gestational age

A missed dating scan will leave you clueless about the exact gestational age of the fetus. NT scan will tell you the age of the fetus.

Placental position

Whether the placenta is anterior, posterior, or fundal. All three are normal placental positions. Risk includes when the placenta lies near to the os which is termed as low lying placenta or placenta previa if the placenta covers the internal os.