The combined screening test is offered between 11 and 14 weeks and checks the chance of your baby having Down's, Edwards', or Patau's syndrome. It combines an ultrasound measurement of fluid at the back of the baby's neck (nuchal translucency) with a blood test and your age. A higher chance result (1 in 2 to 1 in 150) leads to a choice of NIPT, diagnostic testing, or no further testing.
Nuchal translucency (NT) refers to a small collection of fluid at the back of your baby's neck. Every baby has some fluid there, and it is completely normal. During the ultrasound, the sonographer measures this fluid pocket in millimetres. A measurement up to about 3.5mm is generally considered within the normal range. Higher measurements may indicate an increased chance of a chromosomal condition, a heart defect, or other structural differences, although many babies with a slightly raised NT are born perfectly healthy.
The measurement must be taken when your baby's crown-rump length (CRL) is between 45mm and 84mm, which corresponds to roughly 11 weeks and 2 days to 14 weeks and 1 day of pregnancy. Outside this window, the measurement becomes less reliable, and a different test called the quadruple test may be offered instead.
The combined screening test has three components that are assessed together.
The ultrasound scan measures your baby's nuchal translucency and crown-rump length. This is usually done at the same appointment as your dating scan, so you will likely see your baby on screen for the first time during this visit.
A blood test measures two hormones in your blood: pregnancy-associated plasma protein A (PAPP-A) and free beta-human chorionic gonadotropin (free beta-hCG). Unusual levels of these markers, combined with the NT measurement, help refine the screening result.
Your age is factored into the calculation because the chance of chromosomal conditions increases with maternal age, although babies with these conditions can be born to parents of any age.
A laboratory combines all three factors using a statistical model to produce your individual result.
Your result is given as a chance, such as 1 in 1,000 or 1 in 100. This is not a diagnosis. It tells you the statistical likelihood of your baby having one of the three conditions.
Lower chance result: If your result is 1 in 151 or higher (for example, 1 in 500 or 1 in 2,000), this means the screening suggests a lower chance. No further testing is needed unless you choose to have it.
Higher chance result: If your result falls between 1 in 2 and 1 in 150, this is classified as a higher chance result. About 1 in 20 people who have the combined test will receive a higher chance result. Most of these babies will not have the condition, but further testing is offered.
Receiving a higher chance result can be understandably worrying. Your midwife or a screening coordinator will talk through your options.
Non-invasive prenatal testing (NIPT) is a blood test that analyses tiny fragments of your baby's DNA circulating in your bloodstream. It is more accurate than the combined test, with detection rates above 99% for Down's syndrome. The NHS now offers NIPT following a higher chance combined screening result.
Diagnostic testing includes chorionic villus sampling (CVS), usually offered from 11 to 14 weeks, or amniocentesis, offered from 15 weeks onwards. These can tell you with near-certainty whether your baby has a chromosomal condition. Both carry a small risk of complications.
Choosing not to have further testing is also a valid option. Some parents prefer to continue their pregnancy without additional tests, and your care team will fully support whichever decision you make.
Sometimes the sonographer cannot get a clear measurement of the nuchal translucency. This can happen if your baby is in a difficult position, if fibroids or uterine scarring are present, or if a higher BMI makes imaging harder. If the measurement cannot be taken, you will usually be offered the quadruple test instead, which can be done between 14 weeks 2 days and 20 weeks.
The combined screening test specifically looks at the chance of Down's, Edwards', and Patau's syndromes. It does not check for other chromosomal conditions, single-gene disorders, or structural abnormalities. Your 20-week anomaly scan is a separate, more detailed scan that checks your baby's physical development.
There is nothing specific you need to do to prepare. Some hospitals ask you to arrive with a comfortably full bladder for the ultrasound. The scan typically takes around 20 minutes, although it may take longer if your baby needs to shift position.
You may want to think beforehand about whether you would like to have the screening and what you might want to do if you receive a higher chance result. The NHS screening programme is entirely optional.
Antenatal Results and Choices (ARC) offers a confidential helpline (0207 713 7486) staffed by trained professionals who can talk you through your results and the decisions ahead.
Tommy's provides evidence-based pregnancy information and a free midwife helpline.
Your screening midwife or coordinator at your local hospital is also available to explain results and discuss next steps at any point.
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