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What is the meaning of the Down Syndrome risk evaluation result?

What are the limitations of the screening test?
(biochemical screening and nuchal translucency)

The result of the screening test is obtained as a weighted risk taking your age and the results of the tests you have undergone into account. It predicts the statistical risk: one out of a number of women of the same age having an identical result in her screening test will give birth to a child with Down syndrome.

If a woman receives a result of a 1 in 100 risk, this means that if 100 women of her age report with the same screening test values, and all of them undergo amniocentesis, 99 of them will be found to be carrying a fetus that does not have Down syndrome, and one will be found to be carrying an affected fetus. In other words, there is a 99% probability that the infant is normal as far as Down syndrome is concerned. Moreover, a woman who receives a weighted Down syndrome risk of 1:500 has an even lower risk - 499 women out of 500 with the same values will have infants without Down syndrome and one will have an affected infant. Similarly, a woman who receives a 1:1000 risk will have a 99.9% chance of her fetus not having Down syndrome.

When is the risk considered high according to medical definitions?

A weighted risk in excess of 1:386 is considered high (a result defined as "abnormal").

Women with a high weighted risk are invited to come for genetic counseling, where the results are explained to them and they are advised to undergo invasive diagnostic testing (amniocentesis or chorionic villus sampling, depending on the week of gestation).

Only this will give a precise, final result for whether the fetus has Down syndrome, another chromosomal disorder, or has normal chromosomes.

See also the information sheet: "Why is there no medical recommendation for amniocentesis in all cases?"

Does the recommendation for amniocentesis indicate that the mother has a serious problem?

It is common for the mother to receive an "abnormal" result in the survey test (a risk higher than 1:386 for Down Syndrome), with the fetus being found to be normal in subsequent prenatal testing (e.g. chorionic villus sampling or amniocentesis). In fact, about 98% of the women who receive an abnormal screening test result have a healthy fetus. On the other hand, it can happen that the biochemical screening test (or the nuchal translucency test) can erroneously indicate a relatively low risk for a fetus to have Down syndrome, whereas in subsequent prenatal testing the infant will be found to be affected.

Limitations of the screening test

Considering a risk of 1:386 as the borderline between high risk and lower risk means that the predictive power of the screening test is not 100% absolute, and there is a possibility that children with Down syndrome will be born to a certain percentage of women despite the result of the screening test having been regarded as "normal". Even in the case of a very low risk for Down syndrome, such as 1:5000, there is still a small chance that the fetus will be affected. It appears that if only all the women whose weighted risk for Down syndrome obtained from one screening test (blood biochemical test or nuchal translucency test, separately) were to undergo amniocentesis (or chorionic villus sampling), we would only be able to identify 66% of all the cases of Down syndrome in this country. In this situation, amniocentesis would be performed on approximately 5% of pregnant women in the country. If, however, the borderline for recommending amniocentesis or chorionic villus sampling is taken as more than 1:1000, for example, the detection rate will be higher, but the percentage of women undergoing amniocentesis will increase significantly.

Another limitation of screening tests is their limited ability to predict the statistical risk for chromosomal syndromes other than Down syndrome. Despite the fact that some of these cases (mainly severe chromosomal disorders such as trisomy 18, trisomy 13, etc.) are expressed by an increased calculated statistical risk for Down syndrome, most go undetected.

Most of the cases that go undetected are chromosomal disorders of lesser severity than Down syndrome, and descriptions of these can be found in the information sheets titled "Turner syndrome, Klinefelter syndrome, etc." These chromosomal disorders can also be confirmed or ruled out completely by amniocentesis. The frequency of chromosomal disorders other than Down syndrome is about one in every 300 pregnancies on average - this risk is greater in older women, and over the age of 35 the risk is more than 1 in 100 (more than 1%).

Despite these limitations, the "high risk group" for whom amniocentesis is offered can be defined using the Down syndrome screening test (biochemical screening or nuchal translucency, etc.). Using these tests, the invasive tests are directed to a relatively small group of women, preventing the birth of many babies with chromosomal disorders. It should be noted that these screening tests can also indicate risks for problems other than Down syndrome - e.g. defects of closure of the spinal column, kidney defects, and other chromosomal problems (as specified and subject to the aforementioned limitations), etc.

The future of screening tests for detecting Down syndrome

Today it is possible to calculate the weighted risk taking into account all the tests that the mother undergoes. Recent studies have shown that the statistical data evaluating the risk for Down syndrome can be weighted to obtain a single combined risk. This combined risk has been found to be more reliable than a risk calculated based on a single test. Whereas a single screening test such as measurement of alpha-fetoprotein performed in the second trimester can identify 66% of cases of Down syndrome in the country, and amniocentesis is performed only on those women in the risk group (who have a risk greater than 1:386), the combined risk assessment can result in the detection of over 95% of all cases of Down syndrome without increasing the number of women for whom amniocentesis is recommended. It must be emphasized that the combined risk assessment method is still undergoing evaluation.

In the not so distant future, it should be possible to perform chromosome testing of the fetus by examining the pregnant woman's blood. Between the 5th and the 8th week of pregnancy a blood sample will be taken from the mother. At that stage of gestation the mother's blood contains cells from the fetus that have passed through the placenta into the maternal bloodstream. We already have today systems that can "enhance" (increase the relative concentration of) the percentage of fetal cells in the mother's bloodstream for the purpose of identifying fetal chromosomal problems. The intention is that this test will join the current screening tests both in order to reduce further the number of invasive interventions in pregnancy, and also to identify a greater percentage of problems.

It is anticipated that by the end of the decade the test tube pregnancy (in vitro fertilization) rate will have increased significantly. Then each fertilized embryo will be examined before being returned to the uterus in order to ensure that the embryos that are returned are chromosomally normal. Today at least two in vitro fertilization centers in the USA are carrying out trials in which only embryos that have undergone chromosomal testing are returned to the uterus. The risk of chromosomal disorders in embryos returned to the uterus using this method is less than 1:50,000. The purpose of the trials is to see whether the success rate of in vitro fertilization treatments can be increased. As mentioned, these tests are currently expensive and on an experimental basis only.

For further details, see the information sheet titled: How to reduce risk of Down syndrome.
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Comments (2)

Monday, August 24, 2015 8:24 PM
(1) cocok  says:


Monday, August 24, 2015 8:25 PM
(2) Binsea  says:

Well, it looks like a mini train model, who designed this? It is very delicate.

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