The classic prenatal diagnosis of the first trimester includes the combined test – USG + PAPP-A + free B-hCG subunit. This test can be complemented by a free fetal DNA test (NIPT). This is the new face of prenatal screening – writes Jakub Staniczek, MD, PhD from the Department and Clinic of Gynecology, Obstetrics and Oncological Gynecology of the Medical University of Silesia and the Hospital of the Order of Bonifraters in Katowice.
- The NIPT test is based on the assessment of small fragments of fetal DNA in the blood serum of a pregnant woman
- Although called a non-invasive test, it actually requires minimal tissue disruption by puncturing to collect a blood sample from the expectant mother.
- The origin of the fetal DNA particles circulating in the mother’s bloodstream is not fully understood, but it seems that their main sources are chorion (which will be the placenta in the future), amniotic fluid and fetal cells undergoing apoptosis (programmed death).
- More information can be found on the Onet homepage
The amount of fetal DNA increases as pregnancy progresses. On average, in the 10th week of pregnancy, the fetal fraction is 10-15%. all free extracellular DNA in the pregnant woman’s blood. Occasionally, the level of the fetal fraction is lower and may be associated with female obesity or a genetic disease of the fetus.
NIPT is a screening test
Free fetal DNA in the mother’s blood is detected from the 5th week after fertilization, however commercial tests can be performed from the 10th week of pregnancy. The waiting time for the result is about 2 weeks.
NIPT type tests have been available on the Polish market for almost 10 years and are carried out in significant numbers each year. It should be emphasized that testing fetal DNA is a screening test, not a diagnostic test. In contrast to invasive tests, which are diagnostic procedures, the result of a non-invasive test is a statistical result that provides information about the possible degree of risk of genetic disorders in the fetus, and only of those subject to the test. It must be remembered that an incorrect NIPT result must always be confirmed by an invasive assessment of the fetal karyotype!
- See also: What to do to prevent the gynecological examination from stressing?
Currently, tests are performed for the most common trisomies – trisomy 21 (Down syndrome), trisomy 18 (Edwards’ syndrome) and trisomy 13 (Patau’s syndrome), as well as for sex chromosomes, microdeletion and microduplication syndromes.
The main advantage of evaluating free fetal DNA as a screening test is its very high sensitivity and low false-positive rate. The detection rate for Down’s syndrome is around 99,7%, Edwards’ syndrome is 97,9%, and Patau’s syndrome is 99,0%.
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Analyze the results only with your doctor
NIPT should always be discussed, analyzed, and ordered by a physician experienced and knowledgeable in prenatal diagnosis. It is unacceptable to perform the test without medical control – it should be remembered that the test of free fetal DNA is associated with false positive or false negative results. False positives may be due to the vanishing twin phenomenon, the presence of chromosomal aberration only in the chorionic villus in the form of mosaicism, or an abnormality in the maternal karyotype. The result may also be affected by the pregnant woman’s cancer. On the other hand, the main cause of false-negative results is the too low percentage of the fetal DNA fraction obtained from the pregnant serum.
In Poland, according to the recommendations of the Polish Society of Human Genetics and the Polish Society of Gynecologists and Obstetricians, free fetal DNA testing is most often offered to pregnant women at intermediate risk of trisomy in the fetus after the first prenatal examination, amounting to 1: 100-1: 1000. The test should not be offered to high-risk women as a replacement for invasive tests, as it reduces the detection rate of genetic diseases.
Free fetal DNA testing is a dedicated complement to the first trimester prenatal diagnosis and should not replace the combined test as we would lose the anatomical assessment of the fetus. Moreover, by replacing the biochemical part of a combined test with fetal DNA testing, we would lose the ability to screen for pre-eclampsia (pre-eclampsia) and fetal growth disorders.
To sum up, despite its high price, it is a very useful test for fetal genetic defects, and the correct performance and result is guaranteed by the appropriate qualification of the pregnant woman by a doctor.
The article comes from the educational campaign “Conscious motherhood” prepared by Warsaw Press. All materials can be found on http://www.warsawpress.com/
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