The combined screening test, offered between weeks 11 and 14, measures the fluid at the back of your baby's neck (nuchal translucency) and combines it with a blood test to assess the chance of Down's, Edwards', and Patau's syndromes. It is optional and gives a probability, not a diagnosis. Higher-chance results lead to further diagnostic tests if you choose.
The combined screening test is offered by the NHS between weeks 11+2 and 14+1 of pregnancy, usually at the same appointment as your dating scan. It screens for three chromosomal conditions: Down's syndrome (trisomy 21), Edwards' syndrome (trisomy 18), and Patau's syndrome (trisomy 13).
It is called "combined" because it brings together two pieces of information: an ultrasound measurement of the fluid at the back of your baby's neck (nuchal translucency) and a blood test measuring two pregnancy hormones. Together with your age, these factors are used to calculate a statistical probability.
The test is completely optional. Your midwife will discuss it with you at your booking appointment so you have time to decide whether you want it. There is no right or wrong choice.
Nuchal translucency (NT) is the measurement of a small, fluid-filled space at the back of your baby's neck. All babies have this fluid, but babies with certain chromosomal conditions tend to have a larger-than-average collection.
The measurement is taken during the ultrasound scan. Your sonographer will measure the clear space between the skin and soft tissue at the back of the baby's neck. A typical NT measurement is between 1mm and 2.5mm. Measurements above 3.5mm are considered raised and may prompt further investigation, though a raised NT does not automatically mean there is a problem.
The measurement only works within a specific window (11+2 to 14+1 weeks) because after 14 weeks the fluid is naturally reabsorbed. This is why the timing of the test matters.
The blood component of the combined test measures two hormones in your blood:
Free beta-hCG (human chorionic gonadotropin). Higher-than-average levels can be associated with a higher chance of Down's syndrome.
PAPP-A (pregnancy-associated plasma protein A). Lower-than-average levels can be associated with a higher chance of chromosomal conditions.
The blood test is a simple blood draw, usually taken at the same appointment as the scan or at a separate visit shortly before. Results are combined with the NT measurement and your age to produce an overall probability.
The combined test gives you a probability, not a diagnosis. Your result will be expressed as a ratio, for example "1 in 1,500" or "1 in 150."
Lower-chance result (sometimes called "screen negative"). If your combined result is 1 in 150 or lower (for example, 1 in 800), the screening suggests a lower chance of the conditions tested for. The vast majority of people receive a lower-chance result. No further testing is offered as part of routine screening.
Higher-chance result (sometimes called "screen positive"). If your result is higher than 1 in 150 (for example, 1 in 100), you will be offered further testing. Around 3-5% of people who have the combined test receive a higher-chance result. It is important to understand that a higher-chance result does not mean your baby has a chromosomal condition. Most people who receive a higher-chance result go on to have a baby without the condition.
If you receive a higher-chance result, you will be offered further testing. The options include:
NIPT (non-invasive prenatal testing). A blood test that analyses fragments of your baby's DNA in your blood. It is more accurate than the combined screening test, with a detection rate of over 99% for Down's syndrome. NIPT is now available on the NHS in many areas as a second-line screening test. It is non-invasive and carries no risk to the pregnancy.
Amniocentesis. A diagnostic test (not a screening test) that gives a definitive answer. A needle is used to take a small sample of amniotic fluid, which contains your baby's cells. It is highly accurate but carries a small risk of miscarriage (around 0.5-1%). It is usually performed after week 15.
CVS (chorionic villus sampling). Another diagnostic test where a small sample of placental tissue is taken. It can be performed earlier than amniocentesis (from week 11) and carries a similar small risk of miscarriage.
You do not have to have any further testing. At every stage, the decision is yours. Your midwife and the screening team will give you information and support, but will never pressure you either way.
Sometimes the NT measurement cannot be taken, for example if the baby is in a position that makes it difficult, or if you are past the 14+1 week window. In this case, you may be offered the quadruple test instead, which is a blood-only screening test available between weeks 14+2 and 20+0. The quadruple test screens for the same conditions but is slightly less accurate than the combined test because it does not include the ultrasound measurement.
Deciding whether to have screening is a personal choice. Some parents want as much information as possible. Others prefer not to know, or feel that the result would not change anything for them. Both positions are completely valid.
If you are unsure, your midwife can talk you through the process, what the results mean, and what your options would be at each stage. You can also speak to the Antenatal Results and Choices (ARC) helpline (0845 077 2290 or arc-uk.org) for independent, non-directive support.
Whatever you decide, it is the right decision for you and your family.
The nuchal translucency (NT) test measures the fluid at the back of your baby's neck during an ultrasound between weeks 11 and 14. Combined with a blood test and your age, it calculates the probability of Down's, Edwards', and Patau's syndromes. It is a screening test, not a diagnosis.
The combined screening test is offered between weeks 11+2 and 14+1 of pregnancy, usually at the same appointment as your dating scan. The timing is important because the nuchal translucency measurement can only be taken within this window.
A higher-chance result (above 1 in 150) means further testing is recommended, but it does not mean your baby has a chromosomal condition. Around 3-5% of screened people receive a higher-chance result. Most go on to have unaffected pregnancies.
No, it is completely optional. Your midwife will discuss it with you so you can make an informed decision. Some parents choose screening for information, others prefer not to. There is no right or wrong choice.
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