The combined screening test is performed between the 11th week and the 13th week + 6 days of pregnancy. The fluid thickness behind the baby's neck (NT) is measured via ultrasound, and two hormones in the mother's blood are checked during the same period. This information, along with the mother's age, is calculated by a program to determine the probability of certain chromosomal differences, such as Down syndrome. You will see a ratio like "1/2500" or "1/120" on the report. This is not a diagnosis; it indicates the probability of the baby having one of these conditions.
When you receive the report, most expectant mothers first check the internet, where they often encounter conflicting information. Below, we break down the report line by line and explain what to do if the result is "high risk".
What does the combined screening test look for?
On the ultrasound side of the test, nuchal translucency (NT) is measured. Every baby has a thin layer of fluid under the skin of the neck. If this layer is thicker than normal, it may be associated with chromosomal differences and certain heart problems.
On the blood side, two substances from the placenta are checked. Free beta-hCG is usually higher than expected in pregnancies with Down syndrome, while the protein called PAPP-A is often found to be lower in the same situation.
The program calculates these three values along with information such as the mother's age, weight, smoking status, whether the pregnancy was achieved via IVF, and the exact gestational age. The report includes three separate probabilities: trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome).
When is it done, and why is the window so narrow?
The measurement is performed when the baby's crown-rump length (CRL) is between 45 and 84 mm, which corresponds to approximately 11+0 to 13+6 weeks of gestation. If done earlier, the baby is too small for measurement. After the 14th week, this fluid usually decreases. The risk calculation is defined only for the 11+0 to 13+6 week period.
In practice, the most comfortable measurement is taken around the 12th week. If the baby's position is not suitable, you may need to walk around and try again or schedule another appointment within the same week. This is a common situation and does not indicate a problem.
If you miss this window, the screening is not completely lost. The triple or quadruple test performed between the 15th and 22nd weeks or the cell-free DNA test from maternal blood can fill this gap. However, the additional information provided by the NT measurement can no longer be obtained at that point.
What should the NT measurement be?
A single number like "nuchal thickness should be under 2 mm" is often mentioned, but this is not accurate. As the baby grows, the NT naturally increases slightly. Therefore, the measurement is interpreted according to the baby's size.
| Baby's Size (CRL) | Approx. Gestational Week | Expected Upper Limit for NT (95th percentile) |
|---|---|---|
| 45 mm | 11+0 | approx. 2.1 mm |
| 60–65 mm | around 12+2 | approx. 2.4 mm |
| 84 mm | 13+6 | approx. 2.7 mm |
A value slightly above this limit is not considered a cause for alarm on its own; it enters the risk calculation and affects the result. Measurements of 3.5 mm and above, regardless of the baby's size, are handled separately. In this case, regardless of the blood values, a genetic test and a detailed heart ultrasound (fetal echocardiography) are recommended.
The accuracy of the measurement depends heavily on the person performing it and the device used. The baby must be viewed from the exact side profile, the screen must be sufficiently magnified, and the measurement cursor must be placed correctly. Therefore, it is important that the NT measurement is performed by a physician trained in this area.
What does "1/250" on the report mean?
The ratio shows how many women with the same characteristics and test results as you would have a baby with the relevant chromosomal difference. A result of 1/1000 means that in one out of a thousand pregnancies in this group, the baby will have Down syndrome, and in 999, they will not.
Laboratories compare this ratio to a specific threshold. In many centers, this threshold is between 1/250 and 1/300. If the ratio indicates a higher risk than the threshold (e.g., 1/90), the report will say "high risk". In the opposite case, it says "low risk". For results in between, some centers use the term "intermediate risk" and recommend additional screening.
What do MoM values mean?
You will see values labeled "MoM" next to the hormones in the report. This abbreviation shows how many times your value is compared to the average value for the same gestational week. 1.0 MoM means exactly the average. 0.4 MoM for PAPP-A indicates it is significantly below the average, while 2.0 MoM for beta-hCG indicates it is twice the average. Do not try to interpret these values on their own. What matters is the final ratio calculated from all of them together.
A note: A significantly low PAPP-A result may sometimes be related to how the placenta is functioning, without any relation to chromosomes. In such a case, your physician may want to monitor the baby's growth more closely in the coming weeks.
The risk is high: What's next?
First, know this: In the vast majority of pregnancies that receive a high-risk result, the baby's chromosomes turn out to be normal. Screening tests are intentionally set to be "sensitive" to reduce missed cases. The price of this is that some healthy pregnancies fall into the high-risk group.
At this point, you usually have three options:
| Option | When? | Does it provide a definitive diagnosis? | Miscarriage risk |
|---|---|---|---|
| Cell-free DNA test from maternal blood (NIPT) | From the 10th week | No. It is a very sensitive screening, but a positive result must be confirmed. It may rarely fail to provide a result and does not provide information about the baby's organ structure. | None, only blood is drawn. |
| Chorionic Villus Sampling (CVS) | 11–14th week | Yes | Low; additional risk associated with the procedure is reported at a level of a few per thousand or less. |
| Amniocentesis | After the 15th week | Yes | Low; additional risk associated with the procedure is reported at a level of a few per thousand or less. |
Which one is suitable for you depends on how high the risk is, whether an additional finding is found on ultrasound, and your preference. If the risk is very high or there is a structural finding on ultrasound, it is usually more accurate to proceed directly to a definitive diagnostic test. Cases where the NT is 3.5 mm and above also fall into this category, as these babies are also checked for small genetic changes other than chromosome count (microarray). If there is a borderline risk, NIPT can be a reasonable intermediate step. We recommend that you make this decision together with a perinatology specialist and, if necessary, a genetic counselor.
If the risk is low, does it mean everything is fine?
A low-risk result is very reassuring, but it does not mean zero risk. The combined screening catches about 8–9 out of 10 pregnancies with Down syndrome. It also does not provide information about genetic conditions other than the three chromosomal differences for which the test is designed.
Therefore, the detailed ultrasound (detailed USG) between the 18th and 22nd weeks should not be skipped even after a low-risk result. The baby's organs are evaluated most clearly during this period. The combined test looks at chromosomes, while the detailed ultrasound looks at organs. One does not replace the other.
What else is checked during the 11–14th week ultrasound?
This ultrasound is not performed solely to measure nuchal thickness. The gestational age is finalized, it is determined whether the babies in a twin pregnancy share the same placenta, and some major structural problems can be detected in this early period. If you are having your follow-up in the Perinatology or Obstetrics and Gynecology unit, you can ask in advance whether the measurement and blood draw can be done in the same appointment.
Physician who reviewed the text: Obstetrics and Gynecology specialist Op. Dr. Ömer Altun.
Disclaimer: The content on this page is for informational purposes only. Please consult your physician for diagnosis and treatment.
