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Antenatal screening, Down syndrome test. What is it for, how many weeks, results.

Do you have to take the Down syndrome test and would you like to know what it is for, how to do it and how reliable it is? Mom, read on and you will know all about this important antenatal screening


What is the Down syndrome test?

The Down syndrome test, also called combined test, is a non-invasive prenatal diagnostic test that is performed during the first trimester of pregnancy. It is not a test of certainty, since it does not give a sure and definitive result on the baby's health, but a prenatal screening that only provides an index of risk or probability that the fetus is affected by the disease. The result is the result of processing the data collected by the duo test consisting of:

  • Maternal age

  • Measurement of nuchal translucency

  • Plasma level value of pregnancy-related protein A (PAPP-A)

  • Free beta human chorionic gonadotropin (bhcg) value.

These figures combined will produce an index of the baby's risk of having a genetic disease. Downs syndrome testing allows to achieve an accuracy of 91% in the risk assessment of Down syndrome and 98% for trisomy 18. If there is a positive combined test, you can choose whether or not to confirm the fetal disease with invasive prenatal investigations such as CVS or amniocentesis.

Antenatal screening, Down syndrome test. What is it for, how many weeks, results.

Downs syndrome testing

The down syndrome pregnancy test is a prenatal screening that only indicates if the fetus is at risk of being affected by trisomies 13, 18 and 21 respectively Pateau syndrome, Edwards syndrome and Down syndrome. The sampling data and fetal measurements made with the ultrasound of the bi test in pregnancy are combined and give an index of risk that the fetus is disturbed by morbidity. The exam will provide three types of results:

  • low risk index;

  • medium risk index;

  • high risk index.

The altered risk indices, to confirm the disease, require more detailed fetal tests to determine or not the type of pathology.

When is down syndrome screening done? How many weeks

Down syndrome screening is performed between the first trimester and the second trimester of pregnancy, between the 11th and 14th week of gestation. The test is aimed at all pregnant women regardless of age and high or low risk of fetal chromosomopathies.

How is down screening performed?

Down screening consisting of two different phases:

  • a maternal blood test

  • an ultrasound.

Specifically, with the duo test, two proteins are measured, Free Beta HCG and PAPP-A, and a fetal ultrasound is performed to measure nuchal translucency.

Blood test for down syndrome

The test is based on taking a maternal blood sample to measure two substances

  • B-HCG, free beta fraction of human chorionic gonadotropin;

  • PAPP-A, plasma glycoprotein A associated with pregnancy.

The two substances are of placental origin and in cases where the fetus is affected by Down syndrome the values ​​of papp-a and free beta can be altered. This alteration contributes to increasing the Bitest risk index

Is it necessary to be fasted for blood sampling?

It is not necessary to be fasting to undergo the blood sampling of the duo test.

Translucent ultrasound

In the second part of the screening, the mother is given an ultrasound called nuchal translucency in which the amount of fluids present between the spinal column and the skin in the nape of the fetus is measured using ultrasound. This area is called the nuchal fold of the fetus abbreviated to NT. The increased NT value represents a risk signal that the newborn is sick with trisomy 21 (Down syndrome).

Internal or external scan?

Generally the transnucal involves an external ultrasound performed transabdominally, but in the rare cases where there is no optimal visualization of the fetus, internal ultrasound is performed which involves the insertion of the probe inside the vagina.

How long does the prenatal test for down syndrome last?

The test in total takes approximately 30 minutes. The withdrawal takes only a few minutes The ultrasound requires less than 30 minutes, the time to visualize the fetus ultrasonographically and make the measurements.

How to prepare for the pregnant down syndrome test?

To undergo the pregnant down syndrome test there are no particular prescriptions to follow. Fasting is not necessary for the sampling, while for the echo it may be useful not to pee before the examination so that the bladder is full, favoring a better visualization of the gestational chamber and the fetus.

Is Down's syndrome test reliable?

Altered sampling values ​​(PAPP-A and beta HCG) have a 70% chance of identifying fetuses with the syndrome.

If the blood values ​​of the duo test are evaluated together with the nuchal translucency, they bring the reliability of the test up to 85.90%.

It must be said that being a statistical calculation there is also a small percentage of false positives or false negatives which is around 3%, this means that even if the results of Down's syndrome test give a low risk there is still a small residual risk that the fetus have an illness.

Down syndrome test results

The results of the downs syndrome blood test are the result of a software program in which the data collected through the examination are entered together with the maternal age. The data is processed by the computer generating a mathematical result called the risk index. The resulting risk probability is compared with a pattern in which there is a line called cut off. The results of the bitest

  • below the cut off indicate a low risk

  • near the cut off but if always below they express an intermediate risk

  • above the cut off indicate another risk.

It has been conventionally established that the risk of having a baby with Down syndrome is

Reduced if it is less than 1: 350 example 1: 1000 Increased if greater than 1: 350 example 1: 200

The Down syndrome test is not a test of certainty but only a screening test that determines a possibility of risk therefore to make a certain diagnosis it is necessary to undergo an invasive prenatal examination such as villocentesis or amniocentesis which through the karyotype say with certainty whether the fetus has a chromosomal disease.

How soon is the result?

On average, the test report is available after seven days. The outcome of the prenatal test should always be discussed with your midwife or your gynecologist.

Low-risk or negative Down syndrome test

A risk probability of less than 1: 350 is considered low risk, therefore, the expectant mother is not directed to invasive screening tests as long as the couple of future parents want to be absolutely sure of the absence of fetal chromosomal diseases.

High-risk or positive Down syndrome test

In the case of an altered duo test, the fetus could be affected by Down syndrome or other chromosomal alterations. Normally in cases where there is a fetal disease, the serum concentration of HCG is higher than normal while the PAPP-A substance is lower than normal levels. In cases of suspected Down syndrome, the thickness of the nuchal fold is greater than normal. In conclusion, an alteration of these values ​​together with a positive bi test may suggest that the fetus is suffering from a disease that can only be diagnosed with more in-depth tests such as CVS or amniocentesis

What to do if the down syndrome test is high risk.

In the presence of impaired downs screening, health professionals recommend further confirmatory tests to the pregnant woman. The execution of more invasive examinations is not mandatory and depends only on the will of the parents to investigate the clinical case. The in-depth exams consist of:

  • Amniocentesis

  • Villocentesis

  • Fetal DNA test or Nip test

If there is an increased risk, amniocentesis and CVS are free and fully paid for by the national health system, while the Nip test is not a loanable test.

Villocentesis and down syndrome test

CVS is an invasive prenatal examination that is performed between the 11th and 13th week of pregnancy.

A small sample of chorionic villi is taken through the villus, which will then be analyzed to make a chromosomal evaluation of fetal well-being. Being an invasive exam it is not without risks.

Amniocentesis and down syndrome test

Amniocentesis is recommended in cases of high-risk bi-test to carry out a study on fetal chromosomes, arriving at a certain diagnosis of fetal disease. The sampling is performed starting from the 16th week of pregnancy and being an invasive procedure has a slight risk of abortion and rupture of the membranes.

Fetal DNA (nip test) and down syndrome test

In cases of intermediate risk it is possible to perform fetal DNA. It is a non-invasive test (a simple blood sample from the mother) which is performed between the 10th week and the 12th week of pregnancy and allows to detect the fetal DNA circulating in the maternal blood. The test has a 99% reliability for Down syndrome but much less for other trisomies, therefore, in cases where there are multiple risk factors for a possible fetal malformation, CVS and amniocentesis are the most reliable investigations.

Mammamather pocket obstetrician

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