What is the combined screening test and how is nuchal translucency assessed? Screening versus diagnosis, what the result means and options after high risk.
The combined screening test is a screening method in which the nuchal translucency measured on ultrasound in the first trimester of pregnancy and measurements made in the mother's blood are assessed together to estimate the likelihood of some chromosome anomalies, such as Down syndrome, in the baby. Screening is an estimate of risk and does not make a diagnosis: a high-risk result does not mean that the baby has an anomaly, and a low-risk result does not mean that an anomaly has been completely ruled out. Whether to have the test is the person's own decision. According to the current medical view, all pregnant women should be offered both screening and diagnostic test options; you can accept or decline them.
What does first-trimester screening aim at?
First-trimester screening tries to estimate beforehand the likelihood of some chromosome anomalies in the baby. The aim is not to give a definite answer but to help identify in which pregnant women further assessment can be discussed. For the same reason the result is reported not as "present" or "absent" but as a likelihood.
What else can be assessed on the same ultrasound?
The ultrasound in this period assesses, besides nuchal translucency, the week of pregnancy, the estimated due date, whether the pregnancy is single or multiple, and development. The scope of the screening and which measurements will be made may vary according to your doctor's plan.
What is nuchal translucency?
Nuchal translucency is the measurement by ultrasound of the fluid-filled space at the back of the baby's neck. This measurement does not make a diagnosis on its own; it is only one of the data that go into the risk calculation. The measurement result is not interpreted alone; it is considered together with the measurements in the mother's blood. Asking your doctor what a measurement value in your report means is a more reliable way than looking at tables on the internet.
How are ultrasound and blood measurements combined in the combined test?
The combined test rests on assessing together the nuchal translucency measurement on ultrasound and some pregnancy hormones and proteins measured in the mother's blood. This approach is also called the "combined test". The risk is calculated by assessing the ultrasound measurement and the blood result together.
What do low risk and high risk mean?
The result is generally reported as "low risk" or "high risk". High risk means that the likelihood of an anomaly is above the limit set in the screening; it does not mean that the baby has an anomaly. Low risk shows that the likelihood is low; but it does not show that an anomaly has been completely ruled out. The doctor interprets with you what the number given in the result means.
What does a screening test show, and what does it not show?
Every screening test has limits. It is possible for the risk to come out high and the baby to be healthy, and for the risk to come out low and an anomaly to be missed. Current recommendations deal with the benefit, performance and limits of each option separately and emphasise that no single test is the option to be preferred in every situation. For a definite diagnosis, diagnostic tests are needed, not screening; the screening result only guides whether these tests will be discussed.
Is it compulsory to have the test? For whom is it considered?
No. Screening is optional; this is clearly stated. Whatever your age, you can choose to have the test or not to have it. What influences the decision varies from person to person: some people want to learn the results in order to prepare for the pregnancy, while others may not want to receive this information. Both choices are valid. Before deciding, it is useful to talk with your doctor about what the test measures, its limits and what the result will change for you.
What happens if the result comes out "high risk"?
A high-risk result is not a diagnosis; what to do afterwards is a decision that you and your doctor make together. Generally, how the result is interpreted is discussed, and information is given about additional ultrasound or diagnostic test options (such as amniocentesis and chorionic villus sampling). You do not have to have any of these tests; you can ask for information about what each is for, when it can be done and its limits. Feeling anxiety, uncertainty or indecision in this process is usual.
What is the difference between the combined test and the fetal DNA test (NIPT)?
While the combined test rests on the ultrasound measurement and the hormones and proteins in the mother's blood, cell-free DNA screening (NIPT) examines fragments of fetal DNA in the mother's blood. Current recommendations state that cell-free DNA screening can be offered to all pregnant women regardless of age and baseline risk; but this does not mean that it is the option to be preferred in every situation. Which test is suitable for you is assessed by your doctor together with your history and preferences.
How can you prepare for the consultation, and what can you ask your doctor?
- If an anomaly was diagnosed in your earlier pregnancies or there is a chromosome anomaly in your family, tell the doctor.
- If you had IVF treatment and embryo genetic testing was done, share this information.
- Ask which screening options are considered for you, and what each measures and its limits.
- Find out how the result is reported and when and by whom the results will be explained.
- Discuss beforehand which of your decisions the result will influence and what can be done if it comes out high risk.
Frequently Asked Questions
I missed the suitable period for screening; can the combined test still be done?
When the suitable period for screening has passed, the combined test cannot be done; instead, an option done in the second trimester and based only on a blood test can be offered. This option is known to be less accurate than the combined test. The doctor who follows you knows which options are left for you.
If genetic testing was done on the embryo in IVF, is screening still needed in pregnancy?
Genetic tests done on the embryo and screening tests done in pregnancy are separate subjects. The doctor who follows the pregnancy and also knows the result of the embryo test assesses which screenings will be considered in pregnancy; you can raise your question at the first consultation.
The test result worried me a great deal; what can I do?
Patient-centred counselling and informed decision-making are recommended. To understand your result, you can ask your doctor to explain the result in plain language, write down your questions and take them to the consultation, and ask for time to make your decisions without hurrying. You can also ask whether additional counselling, for example genetic counselling, is needed.
Does the combined test make a diagnosis?
No. It makes an estimate of likelihood for some chromosome anomalies. For a definite diagnosis, diagnostic tests, which differ from screening, are needed.
Does a high-risk result mean the baby definitely has an anomaly?
No. It shows that the estimated likelihood is above the limit set in the screening. You discuss the next options with your doctor.
Does a low-risk result completely rule out anomalies?
No. It shows that the likelihood is low; it is possible for an anomaly to be missed. The limits of the test are taken into account when interpreting the result.
Can nuchal translucency be interpreted on its own?
It does not make a diagnosis on its own and is one of the data in the risk calculation. It is assessed together with the measurements in the mother's blood; you should talk about your report with your doctor.
Is it compulsory to have screening?
No. Whatever your age, you can accept or decline the screening or diagnostic test options offered. Beforehand you can learn what they measure and their limits.
After high risk, are amniocentesis or chorionic villus sampling definitely done?
You do not have to have these tests. You can get information about additional ultrasound and diagnostic test options and decide the next step together with your doctor.
Do the combined test and NIPT rest on the same measurements?
No. The combined test brings together the nuchal translucency by ultrasound and the hormone and protein measurements in the mother's blood. NIPT, on the other hand, examines fragments of fetal DNA in the mother's blood.
Does NIPT being offered to all pregnant women show that it should be preferred by everyone?
No. The suitable option is assessed together with your history and preferences. No single test is the option to be preferred in every situation.
Can the week of pregnancy and a multiple pregnancy also be assessed on the same ultrasound?
Yes. The week of pregnancy, the estimated due date, whether the pregnancy is single or multiple and development can also be assessed. The scope of the examination is determined by your doctor's plan.
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