The decision to undergo a screening test is chosen by the patient and does not constitute an obligation.

 

There are screening tests that allow you to identify the risk, that is, the probability that the fetus is affected by some specific genetic problems. These tests are more accurate than the simple maternal age data.

In particular, the following tests exist:

• Combined or ultrascreen test: It is performed between 11 and 13 weeks. It is based on the measurement of the nuchal translucency of the fetus (collection of fluid present behind the neck of almost all fetuses of that gestation age) performed by ultrasound by a certified operator and a blood sampling. Two substances produced by the placenta are sought in the blood: PAPP-A and free β-hCG (the patient does not need to be fasting). By combining nuchal translucency, PAPP-A, free β-hCG, and maternal age, the risk of trisomy 21 (Down syndrome), 13 (Patau syndrome) and 18 (Edwards syndrome) is obtained. It has a sensitivity of around 90% for fetuses with Down syndrome.

• Testing fetal DNA circulating in maternal blood (cell-free DNA). It is performed by taking a maternal blood sample (the patient does not need to be fasting). It has been shown that starting from the first trimester of pregnancy, free fetal DNA (cell-free fetal DNA, cffDNA) is present in the maternal bloodstream, which can be recovered non-invasively through a simple blood sample. This analysis provides the risk of trisomy 21 (Down syndrome), 13 (Patau syndrome) and 18 (Edwards syndrome). It can also determine fetal sex and establish the risk of 5 microdeletions (absence of a small fragment of a chromosome which determines DiGeorge syndrome, Angelman syndrome, Prader-Willi syndrome, Cri-du-chat syndrome, and Deletion syndrome 1p36). It has a sensitivity of approximately 99% for fetuses with Down syndrome.

 

 

If the screening test is "positive" (ie it presents a high risk of Down syndrome), diagnosis can be made by chorionic villus sampling or amniocentesis. On the other hand, if the test is "negative" (low risk) further investigation is not needed, although that does not mean that "certainly" the fetus will not be sick.

 

The diagnosis of chromosomal anomalies can currently be carried out only through the use of invasive techniques such as the withdrawal of placental tissue (chorionic villous sampling) or the withdrawal of amniotic fluid (amniocentesis) following which an abortion can occur in 0.5 -1% of cases.

 

Even in the event of a negative test result, there is a residual risk that the fetus is affected by Down syndrome and that only the execution of invasive tests such as chorionic villous sampling or amniocentesis allows excluding with certainty in the prenatal period a chromosomal anomaly of the fetus.

 

 

Sources and acknowledgments

 

Tratto da informative all'ecografia della SIEOG

Linee guida SIEOG 2015

Linee-Guida Screening prenatale non invasivo basato sul DNA (Non Invasive Prenatal Testing – NIPT) Ministero della salute