Prenatal tests are screening tests performed during the first and second trimester of pregnancy to detect congenital diseases in the foetus.
Katarzyna Mikołajczyk, Doctor of Medicine: Non-invasive prenatal testing is performed first. These are screening tests designed to assess the risk of genetic diseases in the foetus. It is important to remember that a non-invasive prenatal test is not yet a diagnosis, but only a risk, the sensitivity of which depends on the type of test (prenatal test) used. And thus, in the basic FMF ultrasound examination (11-14 weeks of pregnancy), the risk of Down, Edwards and Patau syndromes in the foetus is determined on the basis of the mother’s age, the nuchal translucency of the foetus (NT) and the foetal heart rate. The sensitivity of this test, and therefore the detection rate, is 80%. Ultrasound examination also allows assessment of the basic anatomy of the foetus and detection of so-called large defects. What’s important, some malformations are visible only in the second trimester of pregnancy (e.g. agenesis of the corpus callosum, defects of the heart, kidneys, spina bifida), hence the need for another thorough ultrasound examination between 18-22 weeks of pregnancy. Combining an ultrasound between 11-14 weeks of pregnancy (according to the FMF) with a double test (maternal blood test, PAPP test) increases the sensitivity of the test to about 92%. The result obtained is an information about risk, not a diagnosis. It is generally accepted that if the risk is greater than 1:250-1:300 (i.e. population risk), this is called high risk and invasive diagnostics (e.g. amniocentesis, chorionic villus biopsy) should be considered in order to establish a definitive diagnosis – i.e. confirmation or exclusion of a genetic defect in the foetus.
Another modern screening test is cell free foetal DNA test (cffDNA, NIPT e.g. NIFTY test, HARMONY test) – this test is based on detection and evaluation of circulating in the mother’s blood, free foetal genetic material (DNA), which appears in her bloodstream from the 5th week of pregnancy and disappears shortly after birth. However, to obtain enough foetal genetic material, the test should be performed after the 10th week of pregnancy. The sensitivity of the test is 99.9% for determining the risk of Down syndrome, Edwards syndrome and Patau syndrome. It also allows (although with lower sensitivity) to detect genetic sex – which is of great importance in families burdened with sex-linked genetic diseases, abnormalities in the number of sex chromosomes (Turner’s and Klinefelter syndromes) and much less common microdeletions or microduplications (e.g. DiGeorge syndrome). The patient receives the following result: low or high risk of a specific defect, which, as with the double test, is an indication to extend the diagnosis with invasive testing. It is important to note that this method is also subject to false positives and negatives and is not a definitive result, although it has the highest sensitivity of any non-invasive prenatal screening test available on the market.
With regard to twin pregnancies, the sensitivity of the methods described is lower and blood tests are not applicable in pregnancies with a greater number of foetuses (in such cases it is possible to assess the risk of genetic defects only on the basis of ultrasound markers according to FMF between 11th and 14th week of pregnancy).
Thus, the sensitivity of the PAPP test for a twin pregnancy is about 70-84% depending on the type of pregnancy, and the sensitivity of the free foetal DNA test is 93.7%.
In conclusion, it is important to be aware that more than 90% of foetal abnormalities are found in pregnancies without any family history, and about 70% of children with Down syndrome are born to mothers under the age of 35, i.e., outside the high-risk group. The incidence of chromosomal aberrations (Down, Edwards and Patau syndrome) is about 9 per 1,000 live births. Hence, the need to make prenatal testing universal and available to all pregnant women regardless of age and family history seems obvious. Every woman has the right to know if her child has a genetic condition. She has the right to make an informed decision about whether she wants to continue such a pregnancy and be able to prepare for the birth of a sick child. On the other hand, one should be aware that none of the tests presented here is able to rule out all possible abnormalities in the foetus (only the most common ones for which tests are available), and each method has its own specific sensitivity. This means that each method has a risk of false positives (where the baby does not actually have the defect but the results indicate this) and also false negatives (where the test result indicates a low risk but the baby is born ill). However, the higher the sensitivity of the test, the lower the percentage of false results.
A large second group of genetic tests are invasive tests – such as classic cell culturing method, rapid tests for aneuploidy, microarrays, tests for monogenic and metabolic diseases. These are, however, highly specialized tests, tailored to the needs of specific patients, after a thorough genetic analysis, indicated only in a narrow group of patients at risk.