First Trimester (Combined) Screen for Down’s Syndrome and other fetal anomalies
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
First trimester combined screening (Down’s syndrome screening) combines a blood sample from the pregnant person with an ultrasound scan measurement of the fetus to assess the chance of certain chromosomal conditions. It is used to estimate the likelihood that the fetus has Down’s syndrome (trisomy 21) or other chromosomal abnormalities early in pregnancy.
Why get tested?
To assess the chance of a fetus having a chromosomal abnormality, such as Down’s syndrome (trisomy 21).
When to get tested?
Usually between 11 and 14 weeks of pregnancy.
Sample required?
A blood sample taken from a vein in your arm. Measurement of nuchal translucency (NT) requires a special ultrasound examination to be performed.
Test preparation needed?
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.
What is being tested?
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 chance of carrying a baby with Down’s syndrome (trisomy 21) or other rarer fetal anomalies like Edward’s syndrome (trisomy 18) or Patau’s syndrome (trisomy 13). Analysing biochemical markers together, along with the woman’s age, increases 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.
Where first trimester screening results indicate an increased chance that a pregnancy is affected by Down’s syndrome or another fetal anomaly further testing will be offered, such as Non-Invasive Prenatal Testing NIPT, Chorionic Villus Sampling (CVS) or Amniocentesis.
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.
Common questions
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 chance that the fetus a pregnant woman is carrying has Down’s syndrome (trisomy 21), or another fetal anomaly, such Edward’s syndrome (trisomy 18) or Patau’s syndrome (trisomy 13).
The test must be performed between 11 and 14 weeks of pregnancy. For women who miss this window there is the option of second trimester screening.
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 chance or probability that a pregnancy will be affected by Down’s Syndrome, or other fetal anomales.. If the chance value is higher than a specific cut-off value (e.g., chance of 1 in 150 or higher), then it is considered that the woman may be at increased chance of having a baby with a fetal anomaly.
In pregnancies with a Down’s syndrome fetal PAPP‑A levels tend to be lower, HCG levels tend to be high and the NT measurement is likely to be high.
In pregnancies with an Edward’s or Pataus syndrome fetal PAPP‑A and HCG levels both tend to be low and the NT measurement is likely to be high. 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 fetal abnormalities and that screening will not detect all cases of Down’s syndrome, Edward’s syndrome or Patau’s syndrome.
For example: 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 fetus does not have Down’s syndrome).
If a screening test is high chance, more definitive tests are needed to confirm a diagnosis. This is usually Non-Invasive Prenatal Testing (NIPT) and if necessary invasive diagnostic tests such as chorionic villus sampling (CVS) in the first trimester or amniocentesis in the second trimester. Whilst CVS and amniocentesis give a definite result, they are also invasive and carry a small risk of miscarriage and a rare risk of injury to the fetus.
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 fetus has not been accurately determined, the results may be either falsely high or low.
In twin pregnancies it is still possible to perform first trimester combined screening using PAPP‑A and hCG, although the accuracy of the test is lower.
In triplet pregnancies it is not possible to use combined screening and chance has to be calculated using ultrasound measurements alone.
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 chance of having a child with Down’s syndrome or other chromosomal abnormality increases with the age of the mother. Although the chance 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.
See the NHS choices links below for more information on Edward’s and Patau’s syndrome.
There is a newer test called Non-Invasive Prenatal Testing (NIPT) which involves analysis of cell-free fetal DNA (cffDNA) from the mothers blood..