Fetal DNA circulating in maternal blood is the genetic material that comes from the placenta. Its dosage allows to detect defects related to chromosomal alterations such as:
- trisomy 21 (down syndrome)
- trisomy 18 (Edwards syndrome)
- trisomy 13 (Patau syndrome)
The presence of free fetal DNA is detectable in maternal blood from 4-5 weeks of gestation, but in most pregnancies the level needed to perform the test reliably is only reached at 10 weeks of gestation.
The exam is part of the non-invasive prenatal diagnostic tests which aim to provide an estimate of the risk of certain chromosome abnormalities.
The test is based on measuring the quantities of specific DNA sequences for each chromosome. For example, to assess the risk of Down syndrome, characterized by an excess of chromosome 21, the fetal DNA sequences of chromosome 21 are examined, their quantity is assessed and compared to those identified in the mother's blood for the same chromosome. . If the test reveals the presence of more DNA sequences in the fetus than in the mother, the result indicates that the fetus may have Down syndrome.
The test consists of a normal maternal blood sample. The exam is recommended for women considered at risk of chromosomal alterations (for example the so-called primiparous in old age, i.e. women who become pregnant at an age no longer very young, after the age of 35).
The test is reliable in identifying a fetus at risk of trisomy 21, less so for the diagnosis of other trisomies such as 18 or 13.
For checks relating to Down syndrome, which is the main indication, the exam may show three possible results:
- positive, the fetus may have Down syndrome. In this case, it is advisable to confirm the result using a more reliable test, such as amniocentesis
- negative, it is highly unlikely that the fetus is affected by Down syndrome
- inconclusive, the result does not provide information. This type of result, which affects approximately 4% of the tests performed, is usually due to the fact that the percentage of fetal DNA present in the collected sample is not sufficient to provide a reliable result. In this case the test can be repeated
All pregnant women can use this test, but it should be noted that today it is only offered privately. The Ministry of Health, while considering it of great interest, has not yet included it in the Essential Levels of Assistance (LEA) provided for by the National Health Service because not the whole national territory has public laboratories capable of guaranteeing its execution to the entire population. .
Ministry of Health. Guidelines Non Invasive Prenatal Testing (NIPT)