There are a number of different tests you may be offered when you are pregnant. And when the doctor or midwife explains what they are it can be hard to absorb all of the vital information. So we’ve written all the need-to-know facts down for you.
What is a Nuchal Scan?
A Nuchal scan is a scan that measures the baby’s nuchal translucency and is combined with a blood test to assess the risk of an abnormal pregnancy, in particular the chance of the baby having a chromosomal abnormality like Down Syndrome or heart defects. Nuchal translucency is a collection of fluid that forms under the back of an unborn baby’s neck. All babies will have this but an excess amount may be an indicator of a developmental problem.
A Nuchal scan looks at the risks of genetic abnormalities such as Down, Edward and Patau syndromes. The scan will also perform essential basic anatomy tests and measure the size of the baby to check if growth as well as the development of the pregnancy is on-track.
If a baby has an extra chromosome such as in trisomy 13, 18 or 21, this causes a range of health problems including mental handicap, cardiac and other major abnormalities and some are lethal. A trisomy is an extra chromosome, and babies who have these often also have an excess of fluid in the base of their necks in the nuchal fold. This is how a Nuchal scan is able to calculate the risk of such genetic abnormalities.
This test should be done between 11 and 14 weeks of pregnancy or when your baby is between 45mm and 84mm in length from crown to rump.
What Happens During the Scan?
This is a non-invasive ultrasound and cannot harm your unborn child in anyway. It is usually done on the stomach but is occasionally done as a vaginal scan for a clearer picture. The image of your baby will be brought up on a screen and the width of the nuchal translucency will be measured (the black bit under the white line of skin at the back of their head). A blood test is often taken afterwards for improved accuracy of results.
How Are Results Calculated?
The results of the scan are determined by combining the nuchal fold measurement, your age and the baby’s gestational age. A computer programme is often used to plot these points and calculate the risk. Your age is a factor as the risk of a chromosomal abnormality increases as you age. And in most cases, the thicker the nuchal fold at certain stages of gestation the higher the risk.
These results may take a few days to get back to your sonographer or doctor.
How Reliable is a Nuchal Scan?
A Nuchal scan has 95% accurate combined screening and therefore cannot definitely diagnose a chromosomal abnormality just the risk. For a more definitive answer, a CVS or amniocentesis will need to be carried out. These second tests are much more risky than a Nuchal scan. Take a look at our other blog posts on this for more information.