How to interpret the results I have obtained

Click on this link, where you will find a risk calculator specifically created so you can corroborate the information you have received regarding the risk that your pregnancy may present trisomy. Enter your data in the corresponding boxes, follow the instructions provided and carefully read the complementary information for a correct interpretation of results.


How can I assess the results obtained in the screening of Down's syndrome?

In order to assess the results you have obtained, it is important that you first read all the information provided in this section (prenatal diagnostic techniques) and based on this information you take the following guidelines into consideration:

1.- The risk that a foetus may carry a chromosomal anomaly is calculated on the basis of the risk that the pregnant woman presents due to her age at the moment of delivery modified by the result obtained with biochemical and/or ultrasound markers.

This table indicates the risk of trisomy 21 at the moment of delivery based EXCLUSIVELY on the mother's age.

Mother's Age Risk of T 21 at delivery
18 1 in 1562
20 1 in 1538
25 1 in 1380
30 1 in 966
35 1 in 428
38 1 in 215
40 1 in 129
42 1 in 75
44 1 in 43


2.- The values of the biochemical or ultrasound markers vary in the different weeks of gestation. In order to compare results and assess them separately from gestation length, when calculating risk, these value are expressed in MoM (multiples of the median). If your blood test is not expressed in MoM, ask your laboratory to perform the conversion.


3.- Overall, the normal value for different markers is 1 MoM.

Therefore, the farther away from 1 MoM the marker's result is, the worse the result is.
This table shows reference values, expressed in MoM. A marker is considered suspicious if its values are greater (>) or lower (<) than these reference values.

Marker MoM
TN >1.8 - 2
PAPP-A < 0.4
Beta hCG libre o hCG <0.4 o >2.5
AFP <0.4 o >2.5
uE3 < 0.5
Inhibina A > 2.5

NT (nuchal translucency).

This is the most important ultrasound marker and the most influential on risk calculation. No pathologies associated with a NT below 1 MoM are known. The thicker the NT is, the worse foetal prognosis is. Pathological values of NT usually range between 1.8 - 2 MoM or a measure greater than 3 mm (regardless of the MoM).

Be careful not to confuse nuchal translucency with nuchal fold. Nuchal translucency and nuchal fold are different parameters and their characteristics are described in question 36 of the website.

PAPP-A. (pregnancy-associated plasma protein A).

Overall, foetuses with chromosomal abnormalities have low PAPP-A values. Values lower than 0.4 MoM increase the risk of chromosomal anomaly.

Free beta hCG or hCG (total human chorionic gonadotropin or its free beta fraction). Foetuses with chromosomal abnormalities can present altered free beta hCG or hCG values. Overall, in Trisomy 21 these values are higher, those higher than 2.5 MoM indicating a possible pathology. In Trisomies 13 and 18, these values are generally low, with suspicious values being those below 0.4 MoM.

AFP (alpha-fetoprotein).

High levels of AFP, higher than 2.5 MoM, may indicate foetal malformation (spina bifida, anencephaly, etc.).

Low levels of AFP, lower than 0.4 MoM, are observed in certain chromosomopathies. Hence, in trisomies 21 and 18 AFP may present low values whereas in trisomy 13 it can be slightly higher than normal.

uE3 (unconjugated estriol).

Values lower than 0.5 MoM can indicate possible chromosomopathy of chromosomes 21, 13 or 18.

Inhibin A

When its values are greater than 2.5 MoM, they may be indicative of trisomy 21 or 13.


These are reference values, but in order to calculate the probability of chromosomal anomaly or risk the different markers expressed in MoM must always be combined with the risk for the mother's age.

That these markers are part of DETECTION tests and not DIAGNOSTIC tests, so even if a value or marker appears to be pathological, it does NOT mean that a chromosomal abnormality has been diagnosed, given that these tests are NOT diagnostic.

These tests only INFORM US ON THE POSSIBILITY that there may be some type of risk. Hence, once you have received your results, these must be assessed by your physician to determine if it is necessary to perform other complementary tests (chorial biopsy, amniocentesis, etc.), given that he has your complete clinical history to aid him in adequately interpreting and correlating these data.

Thus, if you are feeling anxious or if there is some information you do not understand or that is concerning you, try to CALM DOWN and ask for an earlier appointment with your physician. Let us insist on the fact that these are not diagnostic tests, but rather detection or tests to rule out any suspected anomaly.


NUANCES of these types of tests:

4.- Why can I have different levels of Risk if screening is carried out in two different laboratories in a short period of time (1 week).

Because the computer program used to calculate risk is not the same in both centres or biochemical markers are not identical..

Each computer program, used to calculate risk, has reference values (population parameters, etc.) and each marker has medians (used to calculate MoMs) that may vary and that influence risk results, causing these differences.

If the difference were significant, we would have to contemplate the possibility that one of the two screening systems had not been correctly configured.

In cases of discrepancy, screening must not be repeated to "check" results given that they are statistical estimations and not true diagnoses. If an error is suspected, talk to your doctor or with the laboratory head.

5.- Are these results equally reliable in the case of twins or triplets? Can the fact of these twins or triplets being monozygotes or hetozygotes affect the results?

These results are very reliable in the case of twins (mainly if they are monozygotes, in other ways, identical but not of much use in the case of triplets.

In heterozygotic twins (different to each other), the combined screening of the first trimester (which includes NT) is much better than the screenings that only include analytical parameters and no ultrasound parameters.

6.- Are these results equally reliable if it is a twin gestation at the beginning and afterwards one of the twins stops growing and developing (evanescent twin)?

Increased levels of human chorionic gonadotropin (hCG and free beta hCG) have been described, specially during the first trimester of pregnancy, due to the presence of an evanescent twin.

In theory, the presence of an evanescent twin can alter alpha-protein levels in maternal serum but not in the amniotic fluid, as these are specific to each amniotic sac and normally the evanescent twin and the living twin are inside different amniotic sacs.

The presence of both foetuses inside the same amniotic sac, even though possible, is not a common occurrence and it poses a very high risk situation in which each case should be assessed individually.

7.- Can the values of these markers be altered if I am a diabetic or I inject myself with insulin?

When insulin-dependent diabetes is not properly controlled, it may sparingly alter the values of biochemical markers (those found in the mother's blood), but if the person responsible for carrying out the screening is informed of this situation, a correction will be made to compensate for such circumstance.

8.- Can the consumption of alcohol or smoking alter the results of these tests?

The results from the biochemical markers used for the screening of trisomies change depending of the number of cigarettes that the pregnant women smokes. This situation should be reported to the doctor, so that, as in the case above, the necessary correction is made to compensate for such circumstance.

No changes have been reported with regard to the consumption of alcohol.

9.- What other causes may change these results?

The pregnant woman's weight and certain racial traits (black, Asiatic ethnicity, etc.) are among the main causes that have the potential to change biochemical markers. Because of this, most screening centres ask pregnant women these and other type of questions in their information sheets so that the necessary compensatory corrections are made to improve the screening results.

10.- Today I had my 12th week ultrasound done. I am very concerned because, even though at the beginning everything was OK, the test recommends that I have an amniocentesis done. I do not understand this and I get very little information, should I be worried?

This means that when the ultrasound was done, you were told that it was normal but subsequent blood tests have shown slightly altered values (beta free hCG, PAPP-A test, etc.). These results might indicate a risk situation and this is why in such cases an amniocentesis is recommended. You must trust your healthcare professional and follow his or her instructions.

For your peace of mind, you should know that it is important that your first ultrasound was normal. In other words, if with the same risk results the first ultrasound you had done had been abnormal, the possibility for the amniocentesis to be abnormal would also be higher.

11.- How do I interpret the percentages I am being given?

The results of a screening test are expressed as a probability of "1 in a given number" for instance, a 1 in 50 probability means that if a mother were to have 50 pregnancies, the foetus in 1 of these 50 pregnancies would be affected (the foetus would have Down syndrome if the screening had been for that specific chromosopathy). If this probability were to be expressed as a percentage (number per cent), then it would be 1 * 100/50 = 2% which is the same and represents a 2% probability of having a child affected with Down's syndrome.

Risk perception is very subjective and personal. It is generally considered that any risk greater than 1 in 250-350 (cut-off level) is sufficiently high to recommend a confirmatory invasive technique (amniocentesis, chorion biopsy etc.). Please remember, the lower the number appearing to the right of the risk fraction the greater the risk is; thus a risk of 1 in 50 is greater than a risk of 1 in 100 and much greater than 1 in 1000. 1 in 1000 is usually expressed as 1:1000.

As previously explained, to calculate the risk of a pregnant woman having a foetus affected with trisomy 21, 18 or 13, first the mother's risk based on her age at the time of delivery has to be calculated. Next, the resulting figure (risk) is modified and recalculated based on the information yielded by the different markers of screening method being used.

Let's see an example:

Let's suppose a first trimester combined screening is performed on a women that will be 43.5 years old on the estimated due date who has a risk adjusted by her age of 1 in 50.

The screening has been completed and the result yielded by each marker (beta hCG, PAPP-A and TN) has been 1 MoM. The risk result for trisomy would be 1 in 824 ; that is, much lower than that corresponding to her age and given that the risk is lower than 1 in 250 (cut-off value for the indication of amniocentesis at her healthcare centre), amniocentesis will not be recommended.

If on the contrary, this same pregnant woman had 2 MoM beta hCG, 0.5 PAPP-A and 1.5 MoM TN she would have the same risk for her age, that is 1 in 50; however, since the ultrasound/biochemical markers were somewhat altered, the risk for trisomy 21 would change from 1 in 50 to 1 in 13, and therefore amniocentesis would be rendered appropriate.

An invasive technique (amniocentesis, chorion biopsy, etc.) should be recommended whenever the risk for trisomy 21 or for trisomy 18 or 13 is higher than the cut-off point established at each centre (1 in 250-350) but should never recommended in cases where the risk for trisomy (21 or 18-13) is greater than the risk for the pregnant woman's age.

12.- If the first trimester screening was normal, is it necessary to have the second trimester screening?

No. It is not necessary as the first trimester screening is much more reliable than the second trimester screening.

Given that:

the 1st trimester screening is based on the mother's age, an ultrasound parameter, and two biochemical parameters, it allows us to detect between 80-85% of cases of Down syndrome, with a 5% rate of false positives (no risk-free results and normal fetus).

The 2nd trimester screening is based on the mother's age and on two biochemical parameters. This second screening allows us to detect between 60-65% of cases of Down syndrome, with a 7-10% rate of false positives.

It is only in the so called "Integrated Test" that combines markers from the first and second trimesters that determinations are carried out in both trimesters, but the result is only obtained after the second trimester blood tests are done.

The so called "contingent screenings" are under study and, in some centres they are performed as research. In these screening tests, women are classified into three groups according to the risk score obtained in the combined screening test performed during the first trimester: low risk, high risk and intermediate risk (borderline). This latter group is advised to undergo a screening test in the second trimester the Final Risk of which is obtained by combining the results of the first trimester with the results of the second trimester, as in the Integrated Test.

13.- If the PAPP-A value is low and the beta hCG normal, could it possibly indicate any anomalies in sexual chromosomes?

Some Turner's syndromes present a low PAPP-A with a normal hCG.

14.- Should the ultrasound study (nuchal translucency) and the maternal serum screening be performed on the same day?

No, it is not necessary to have both tests on the same day. Whether they coincide or not will not alter the final result, for as long as these studies are carried out within the time period or gestational weeks as indicated by the obstetrician.

15.- Must the risk be recalculated if the mother's age is incorrect?

Yes. If mother's age is incorrect it must be modified by the real one (ideally her age at the time of delivery with a decimal point that will correspond to the decimal fraction of the months) and the risk should be recalculated once more, given than the mother's age is a very important marker (the most important factor that we have).

16.- Must risk be recalculated if the gestational age is incorrect?

Yes. Gestational age is used to calculate the biochemical markers MoMs and if the gestational age is incorrect also will be incorrect the results obtained in biochemistry MoMs.

17.- What does "risk adjustment" mean in the case of triplets?

It means that some computer programmes calculate the risk for triplets "(by dividing the MoMs by 3)". Nowadays, this test is COMPLETELY UNRECOMMENDED.

At the most, individual risk may be calculated for each of the triplets by means of echographic markers (NT - and CRL - crown-rump length) only, but never by current biochemical markers.

18.- Is it true that beta-hCG is higher in twin gestations?

Yes, beta-hCG values are nearly two-fold those reported in single pregnancies, as with the majority of biochemical markers.
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