Nuchal Translucency Scan (NT)

When can this Scan be done

  • This scan is carried out from 11 weeks and 4 days to 13 weeks and 6 days. The best time (taking into account the small size of the fetus) is having the scan between 12 weeks and 3 days and 13 weeks 6 days. At this stage the fetal organs can also be examined and the gender determined in ± 70% of cases! A trans-abdominal scan is usually performed; however, it may be required to have a closer look vaginally.

The aim of the NT scan

  • To date the pregnancy accurately (if this has not been done yet).
  • To diagnose the type of twin (or other multiple) pregnancy. It is important to know whether each twin has its own placenta, or whether they share a common placenta. If they share a placenta, it is advisable to monitor the pregnancy more closely.
  • To check whether the baby is growing and developing normally. Some major abnormalities can be visible at 12 weeks, but it is much better to have an ultrasound examination at 20 weeks to exclude structural abnormalities.
  • To assess the risks of Down’s syndrome and other chromosomal abnormalities.
  • Your individual risk for this pregnancy is calculated by taking into account your age, measurement of hormones in your blood and the ultrasound findings.

Personalised Risk For Down Syndrome

  • Most babies are normal. On the other hand, each woman, regardless her age, has a small risk of having a baby with a physical or mental handicap. Everybody has a choice to have a risk assessment for chromosomal abnormalities.
  • Babies with Down syndrome and heart failure tend to have extra fluid at the back of the neck. Only sonographers with the “Certificate of Competence from the Fetal Medicine Foundation in London” are provided with the Computer Evaluation Programme to perform this Down syndrome risk assessment. Each woman will be given an estimate of her individual risk for Down syndrome in this pregnancy.

The Factors Assessed at the 1st Trimester Nt Scan are summarised as

  • Age of the mother
  • Amount of fluid behind the neck of the fetus (nuchal translucency)
  • Presence or absence of the fetal nasal bone
  • Fetal heart rate
  • Blood flow through the tricuspid valve of the fetal heart
  • Blood flow through the ductus venosus in the fetal liver
  • Presence or absence of any major structural fetal abnormality
  • Level of 2 proteins in the mother’s blood (free B-hCG and PAPP-A). This blood test is known as the first trimester Downs syndrome biochemistry. The blood test is ideally done at 10 weeks; however, it can be done between 8 weeks and 0 days and 13 weeks and 6 days.
  • With the results of the blood test included in the calculation, the detection rate for Downs syndrome is approximately 95% and without the blood test results, 80%. Not all babies with Downs syndrome will show physical signs on ultrasound and we therefor feel it is important to have the blood test done. At the end of the test a risk number is calculated. A risk number of 1:2 to 1:300 is high risk, 1:300 – 1:2 500 is intermediate risk and a number above 1:2 500 is low risk for Downs syndrome. The higher the number the less likely it is statistically that the baby has Downs syndrome.
  • There are other tests available that have a better than 95% detection rate for Downs syndrome. These tests are also optional.
  • NIPT (Non-invasive prenatal testing) is a DNA test on maternal blood (therefor carries no risk of miscarriage) to screen pregnancies for the most common fetal chromosome anomalies: trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome). Also, the gender of the fetus is determined. The NIPT is highly sensitive and picks up more than 99% of cases of Down syndrome. But it is still a screening test rather than a diagnostic test. Unfortunately, this test is still expensive in South Africa, approximately R6 500 – R7 500.
  • At the current time, karyotype testing in the prenatal setting is quite invasive, requiring amniocentesis or chorionic villus sampling and carries a small risk of miscarriage, approximately 1 in 300. Karyotyping can be done from blood, hair, or any other tissue. However, most karyotyping for medical diagnostic purposes is done on embryonic or fetal cells from unborn babies still in the uterus. The cells are usually collected by one of two methods: amniocentesis (amniotic fluid collected) performed after 16 weeks or chorionic villi sampling (piece of the placenta collected) performed between 12-14 weeks. The detection rate for Down’s syndrome is 99.99% and we therefor call it a diagnostic test.

More about NT Scans

The size of the fetus is about 5-6.3cm and weigh 14g.

Weeks 12-18 are a major, critical period in your baby’s brain development. Avoid any alcohol, tobacco or drugs because these could have long-term effects on your baby. This is also when your baby’s pituitary gland starts working and making its own hormones. Your baby’s placenta is now doing all the important work – filtering oxygen and nutrients to assist your baby’s growth. This week, your baby can suck its thumb. Its eyelids will fully cover the eyes, so they are well protected. Your baby’s intestines are starting to contract and relax, practicing for healthy digestion when they are born.

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