All pregnant women are offered a screening test in the first trimester to give them information on the chance of their baby having an abnormal chromosomal condition like Down’s syndrome, Edward’s syndrome etc; An NT scan is usually done as a part of prenatal screening in combination with blood test to help ascertain the risk to the baby .
NT stands for Nuchal translucency (NT). Nuchal translucency will measure your baby’s nasal bone and the collection of fluid under the skin behind the baby’s neck. The fluid is part of baby’s normal development in the womb in early pregnancy. NT scan aims at measuring the thickness of this fluid collection. Increased levels may indicate abnormalities in the fetus.
The ultrasound pictures are easier to read if you have a full bladder, so your doctor may recommend drinking lots of water about an hour before your appointment. The ultrasound scan is usually done abdominally so it is preferable that you wear comfortable clothing that makes your lower abdomen accessible.
The duration of the scan could typically last from about half an hour to an hour depending on your baby’s neck position. If the baby’s position is not convenient the doctor might ask you to wait for a while before he/she rescans again.
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.
Results from the NT scan would be usually available 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 measure the levels of certain biochemical markers in your blood like Beta HCG, PAPP A (double marker test). The results of the NT Scan and the double marker test will be taken into account for a final report.
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)
-Congenital heart abnormalities
-Increased risk of miscarriages
Please remember that The NT Scan is a screening test and not a diagnostic one. The test only predicts if your baby is at a high risk or low risk of having a genetic condition. As with any prediction the accuracy rate varies. If you combine the NT Scan with blood test (double marker test)the screening is about 85% accurate for predicting Down’s syndrome. If you don’t combine the blood test the accuracy rate drops to 75 %. Many women who have an abnormal first trimester screening go on to have healthy babies.
If your NT report is abnormal and your first trimester combined screening shows that you are at a high risk, then the foetal medicine specialist will counsel you regarding your options. This will include the offer of a diagnostic invasive testing to confirm the presence of a chromosomal abnormality in the baby: either a chorionic villus sampling done between 11th -14th week of pregnancy or amniocentesis which is usually carried out after the 15th week.
There are risks associated with both these procedures, including miscarriage (1%). Although this can be worrying, it is important to remember that an increased NT does not always mean there is always a health problem.
It will be up to you whether or not to have any more diagnostic tests. If the tests show there is a problem with the baby, you will be supported to make a choice that is right for you and your family.
For most women, the checks will assure that the mother and baby are doing well and no further action is required. For a small group of women, there will be an early warning signal that indicates that the doctor may need to request further tests or prescribe some medications for the remainder of the pregnancy to support the mother and the baby. By doing an assessment early in pregnancy, it enables a care pathway to be determined for you and your baby towards a happy outcome.
An NT Scan is a non invasive, safe procedure for both mother and the baby. Some expectant mothers may skip testing to avoid knowing risk factors, especially if the outcome of the test will have no bearing on their decision to continue with the pregnancy.