Screening is a simple and available solution, which determines the possibility (risk) of a disease before the occurrence of symptoms. Examination targeted to a clear manifestation of the disease or preventive measures, which could prevent a full development of the disease, are offered to the patients with a significant disease risk ("screening positive").
In case of a positive result of a screening test in pregnancy (prenatal test) for an inborn defect, by means of blood tests and ultrasound, the mothers are offered a targeted examination of cells of the amniotic fluid (amniocentesis - AMC), placenta (biopsy choria - CVS) or the foetus’ blood (cordocentesis - PUBS). The screening result may also be the reason for a targeted ultrasound examination (superkonsiliar ultrasound) or monitoring in the consulting room for risk pregnancy.
In case of demonstration of the foetus’ affection with a serious incurable disease, the mother may ask for interruption until 24th week.
The screening examination is simple, but not very accurate. A proportion of patients with positive screening is healthy and the result causes only a false alarm. We speak about a false positive. On the contrary, a proportion of the diseased people is not revealed – that is a false negativity of screening.
Prenatal screening is focused on certain serious foetal defects, which mostly originate randomly as a "genetic accident" during conception or in the first few weeks of pregnancy. The most serious of them, originating during conception, is the Down syndrome, whose cause is the supernumerary block of about 250 genes in all the cells of the foetus stored in the chromosome number 21 (trisomy of chromosome No. 21). An example of an inborn foetal defect, which is developed during the first few weeks of pregnancy is the spina bifida.
Typical signs of the Down syndrome (mongoloidism):
An oval, oriental face, with short eye-lid openings and skin folds coming to the inner canthus from the upper lids into a short button nose. The mouth is small, with a protruding tongue. Calvarium is short. There is a single transverse "monkey’s" fissure on the palms. Loose muscles are the cause of the joint looseness. From the internal defects we can bring out cardiac defects, so typical for the Down syndrome (in 40% of cases), and defects of the alimentary tract (in 20%). It is necessary to expect immunity defects, reduced activity of thyroid glands and a higher occurrence of malignant blood diseases. Mental development is always retarded. The life expectancy is over 40 years.
The most frequent cause (in 90 % of cases) is an incorrect ovum division before the fertilization. The ovum is present in the ovaries of a foetus of the female sex already in the first third of its intrauterine development. The ova are thus in fact biological targets for the external hits and this is one of the reasons why the foetuses of older mothers have a higher frequency of accidentally originated genetic defects.
The risk of a birth of a child with the Down syndrome according to the mother´s age:
|Mother's age ||Risk of Down syndrome in birth |
|20 ||1/1500 |
|25 ||1/400 |
|30 ||1/1000 |
|35 ||1/380 |
|40 ||1/110 |
|45 ||1/30 |
Screening for the Down syndrome according to the mother´s age:
Within the age screening all mothers older than 35 years are positive with the risk of 1/380 of a birth of an affected newborn.
Combined screening (multimarker) using the values of biochemical substances in the blood of the mother and the ultrasound examination of the foetus. This screening system is orientated mainly towards the Down syndrome (DS), Edwards syndrome (multiple defects incompatible with life) and the defects connected to damage of skin coverage of the foetus: defects of neural tube and abdominal wall.
- Multimarker screening in 2nd trimester (between 14th – 21st week of pregnancy) becomes a common part of the prenatal care. For the risk estimation of the foetus’ affection, the values of substances in the mother’s blood (markers), as well as her age, are used.
- The principle trio of blood markers, the so-called "triple test", is: alphaphetoprotein (AFP), human chorio gonadotrophin (T-hCG) and the non-conjugated estriol (uE3)
- Recently, an examination of two markers already in 10th week of pregnancy has been introduced: gravidic plasmatic proteine A (PAPP-A) and a free beta subparticle hCG (Fß hCG). Biochemical examination of the substances in mother’s blood in 10th week is completed with the ultrasound examination of the foetus: magnitude of the neck clearing (nuchal transluence of the foetus (NT) and demonstration of the nasal bone (NB), optimally in 12th week.
- Results of all the tests can be integrated into a single result.
- Genetic ultrasound in 2nd trimester: possible to further specify the risk of an inborn defect by a targeted ultrasound examination during 18th - 20th week.
Protocol of screening for the Down syndrome and other defects in five steps:
At the first visit of the prenatal consulting room the mother is informed of the possibility of the inborn defect screening. Mothers over 35 years within the term are recommended to the genetic consultation.
During 10th week, blood is sampled and tested for PAPP-A.
During 12th week the nuchal translucence (NT) is measured by means of ultrasound and the presence of the nasal bone (NB) of the foetus is determined. Mothers with significantly lower level of PAPP-A, higher nuchal translucence (NT) or with the absence of the nasal bone (NB) of the foetus are sent for consultation into the prenatal diagnostic centre.
Within 14th-17th week, in case of the other mothers, an examination of at least alphaphetoproteine (AFP) and chorio gonadotropine (T-hCG) is performed. Typically, there is a reduced level of the AFP and an increased amount of the T-hCG in the blood of mothers with a foetus affected by the Down syndrome. For the final risk calculation of the inborn defect of the foetus all the known results are used (so-called PAPP-A, NT, NB, AFP, T-hCG). The examination of a further screening marker - free estriole (uE3) - may reveal a risk of some rarer defects (for example, Smith-Leml-Opitz syndrome, Edwards syndrome, deficiency of steroid-sulphatasy) or the defects of placenta with a risk of complication towards the end of pregnancy.
The screening is completed with an ultrasound examination in 20th week, focused according to the preceding course of the pregnancy.
A positive result of screening does not mean a certain damage to the foetus. It only means an increased risk. In reality, only every twentieth to fiftieth foetus is affected.