First Trimester (Combined) Screen Down’s Syndrome Screen
A blood sample taken from a vein in your arm. Measurement of nuchal translucency (NT) requires a special ultrasound examination to be performed.
You may be requested to have a full bladder when having the nuchal translucency ultrasound performed. Having a full bladder when you attend for your scan usually makes it easier to see the baby clearly and measure the nuchal translucency.
The first trimester screen is a combination of two blood tests and a special ultrasound scan that are used to assess a pregnant woman's risk of carrying a baby with Down’s syndrome (trisomy 21). Performing and evaluating them together, along with the woman's age, maximisesincreases both the sensitivity and specificity of the screening results.
- Pregnancy-associated plasma protein A (PAPP-A) is a protein produced first by the outer layer of the developing pregnancy (trophoblast) and then by the growing placenta. During a normal pregnancy, levels of this protein increase in the pregnant woman's blood until delivery.
- Human chorionic gonadotropin (hCG) is a hormone produced by the developing pregnancy and then produced in large quantities by the placenta. The Free beta subunit is used in first trimester screening. Levels usually rise rapidly in the pregnant woman's circulation for the first 8 to 10 weeks, then decrease and stabilise at a lower level for the remainder of the pregnancy.
- Nuchal translucency is a measurement made during the ultrasound scan. The ultrasonagrapher measures the fluid collection between the spine and the skin at the nape of the foetus's neck. It is a procedure that requires a specially trained sonographer, proper alignment of the foetus, and careful measurement.
If the results of first trimester screening give cause for concern, diagnostic tests (e.g.amniocentesis or chorionic villus sampling (CVS) will be recommended.
How is the sample collected for testing?
Blood is taken from a vein in the woman's arm. The nuchal translucency ultrasound scan may be performed from outside the abdomen (transabdominally) or the probe may be inserted into the vagina (transvaginally).
Is any test preparation needed to ensure the quality of the sample?
You may be requested to have a full bladder when having the nuchal translucency ultrasound scan performed. Having a full bladder when you attend for your scan usually makes it easier to see the baby clearly and measure the nuchal translucency.
How is it used?
The combination of tests for PAPP-A, free beta hCG and nuchal translucency that are included in the first trimester screen are used to assess the risk that the foetus a pregnant woman is carrying has Down’s syndrome (trisomy 21).
The first trimester combined screen is the recommended screening test for Down’s Syndrome.
When is it requested?
What does the test result mean?
A mathematical calculation using the results obtained from the PAPP-A and free beta hCG blood tests, and the nuchal translucency measurement is used to determine a numeric risk of Down’s Syndrome in the foetus. This risk is compared with an established cut-off. If the risk is higher than the cut-off value (e.g., risk of 1 in 150 or higher), then it is considered that the woman may be at increased risk for having a baby with Down’s Syndrome.
In pregnancies where the foetus is carrying a chromosomal defect, such as the extra chromosome material that results in Down’s syndrome, the levels of PAPP-A are usually decreased, the levels of free beta hCG are significantly increased, and NT (the space at the foetus's neck) is larger than normal.
The interpretation of these results should be provided by a healthcare professional who can explain the meaning of the results and offer choices about further testing and follow up. It is important to remember that screening tests are not diagnostic of foetal abnormalities but indicate a low or increased risk.
While the first trimester screen can correctly identify approximately 85% of women carrying a foetus with Down’s syndrome, about 2-3% of normal pregnancies will have a false-positive result (the test result is positive, but the foetus does not have Down’s syndrome).
If a screening test is high risk, more definitive tests are needed to confirm a diagnosis. These may include a diagnostic test such as chorionic villus sampling (CVS) in the first trimester or amniocentesis in the second trimester. While these two procedures give a definite result, they are also invasive and carry a small risk of miscarriage and a rare risk of injury to the foetus.
Screening will not detect all cases of Down’s Syndrome.
Is there anything else I should know?
Test results are very dependent on nuchal translucency techniques and the accurate determination of the gestational age of the foetus. If the gestational age of the foetus has not been accurately determined, the results may be either falsely high or low.
In twin pregnancies calculation of the risk of Down’s syndrome can be difficult because the amount of PAPP-A and free beta hCG is increased. The nuchal translucency ultrasound, however, is an assessment that is unique to each foetus which can be done independently with ultrasound. Women with a multiple pregnancy should consult their doctor about their options.
What is Down’s syndrome?
About 1 in 700 babies are born with Down’s syndrome (trisomy 21) each year. The condition causes mild to moderate mental retardation and developmental problems and is associated with congenital heart defects, respiratory and hearing problems, leukaemia, and thyroid disorders. Many of the complications of Down’s syndrome can be treated and the lifespan of those affected has greatly increased in recent years. The risk of having a child with Down’s syndrome or other chromosomal abnormality increases with the age of the mother. Although the risk of having an affected baby is significantly greater in those older than 35, the majority of Down’s syndrome babies (about 80%) are born to those under 35 because this age group has the greatest number of children. For this reason, it is recommended that all pregnant women be offered a screening test for Down’s syndrome.
Are there other non-invasive screening tests for these disorders?
There is a newer test called cell-free DNA (cffDNA) that only requires a blood sample from the pregnant woman and can be used to screen for certain foetal chromosomal abnormalities, including Down’s syndrome, Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). It can be performed as early as the tenth week of pregnancy. Invasive diagnostic tests, such as chorionic villus sampling (CVS) and amniocentesis, are still needed to confirm the results. This test is not currently recommended by the UK National Screening Committee.