ad. 17. What is understood by the “risk” of the Down syndrome?
Every woman has her own initial (basic) risk of having a child with the Down syndrome. It increases along with her age. In the 1970s it has been assumed that the age of 35 is a critical age, and it implies a high risk of genetic disorders and constitutes an indication for diagnostic prenatal examinations. Now we know, that basing only on the age of a mother we are able to detect maximum of 30% of babies with the Down syndrome. Why is that? A group of pregnant women at the age of 35 and above comprises just 20% of all the pregnant women. The rest of 80% are younger women who will give birth to 70% of babies with this disorder. Therefore, it has been accepted that screening tests have to include all pregnant women, and the age of a mother shall be one of the factors influencing the final risk of genetic disorders.
Screening tests may be performed on the basis of an ultrasound examination (a measurement of nuchal translucency, among others), a biochemical examination from a blood sample of a pregnant woman (levels of two hormones in blood serum: free subunit β-HCG and PAPP-A) or combine both into a so-called Combined Screening Test based on the age of a pregnant woman, a value of nuchal translucency and values of hormones in a woman’s blood sample. It is best when the examination covers all sorts of ultrasonographic parameters (the ossification of the nose, the function of a tricuspid valve, the flow in a ductus venosus, and possible identified defects). Such a test is characterized by the highest effectiveness in the first trimester. In a report from the test, under the calculation of the risk, in a small print there is a list of all the parameters taken into consideration during calculations. The result of the combined test is a numerical value (fraction) with a comment, qualifying the patient to a group of a low or an increased risk. Patients who find themselves in the group of an increased risk will be proposed a prenatal examination which excludes chromosomal aberrations with 100% certainty.
Let us analyze two example results of screening tests towards the Down syndrome:
1/3,547 – this value tells us that 1 in 3,547 women at the same age, with an identical obstetric history, the same ultrasound image and similar biochemical parameters may give birth to a baby with a disorder. It is a very good result meaning that the probability of having a genetically healthy child is very high;
1/24 – this value tells us that 1 in 24 women at the same age, with identical obstetric history, the same ultrasound image and similar biochemical parameters may give birth to a baby with a disorder. It is not similar to diagnosis, it only indicates that a screening test was not sufficient and there is a need for a prenatal examination, which is e.g. chorionic villus sampling or amniocentesis.