What is a combined nuchal screening test?
A combined nuchal screening test involves an ultrasound scan and a blood test. The test will pick up 80% of pregnancies affected by Down’s Syndrome. The combined test is recommended by the National Down’s syndrome screening committee.
How does the test work?
The combined screening test looks at both the nuchal translucency ultrasound scan and a blood test from the mother to work out a risk figure.
A Nuchal translucency scan (NT) measures the amount of fluid lying under the skin at the back of the baby’s neck. Babies with Down’s syndrome, heart and other chromosomal abnormalities have an increased amount of fluid here.
The scan will also provide you with additional information on multiple pregnancy, accurately date your pregnancy, and will also exclude major abnormalities and miscarriage at this stage of the pregnancy.
The blood test measures the amount of a hormone and a protein that are found naturally in the mother’s blood. These substances have passed from the baby to the mother.
A computer programme uses the nuchal scan measurement, the size of the baby, the mother’s age and the blood results to work out a risk for Down’s syndrome for the baby.
This combined test allows the screening process to be more accurate.
When is the best time to have the combined screening test?
The best time to perform this test is between 11 and 14 weeks of gestation. Women who are unsure of their dates are recommended to have a dating scan before the screening test.
Who performs the scan?
The nuchal translucency scan is performed by an experienced trained ultrasonographer who is certified for nuchal translucency scanning.
What do the results mean?
The results provide you with a risk factor for the chance of your baby having Down’s syndrome. The information you receive from the combined screening test will help you make a decision on whether to have no further tests carried out or whether you decide to opt for a further invasive diagnostic test. It is recommended that you discuss the type of invasive tests with your midwife/doctor before proceeding.
As a guide, if the combined test shows the risk of the baby having Down’s syndrome is relatively high (e.g. one in 150) an invasive test is usually offered.
What to expect during the test
Before a nuchal scan you will need to drink a pint of water so that you attend with a full bladder. The scan is performed on your tummy. If your baby is in a difficult position an internal scan may be needed.
During the scan the pregnancy will be dated, measured and assessed for major physical abnormalities.
If the baby is between 11 and 14 weeks a blood sample will be taken.
If the results show an increased risk you will be offered a consultation with a consultant obstetrician to discuss the results and the next steps.
To have a combined Nuchal screening test, you can self refer or request a referral through your GP or midwife.