Today, Down syndrome (trisomy 21) screening in the baby
during pregnancy is almost routinely performed. For this purpose, a blood test
is made from the mother to assess whether the baby has a high risk of Down
syndrome, and these tests are called intelligence tests among the public. If
the risk is high, amniocentesis is performed to confirm the diagnosis of Down
syndrome (and trisomy 13, 18). Amniocentesis procedure has some risks, although
rare, so remedies have been explored to confirm the diagnosis with a risk-free
method. The examination of free fetal DNA in maternal blood (free fetal dna
analysis in maternal blood) has been developed for this purpose, so the aim is
not to perform invasive procedures such as amniocentesis or CVS, cordocentesis.
Therefore, cffDNA study is called non-invasive prenatal test (NIPT).
What
is Free Fetal DNA?
(Free fetal DNA = cffDNA = Cell free fetal DNA)
Free fetal DNA is a freely detected genetic material that
passes from the placenta to the maternal blood during pregnancy and is not
found in the cell. Fetus intact cells can also be detected in maternal blood,
but these are very rare and not useful for genetic examination. In the mother's
blood, it has been reported that the cells belonging to the baby can remain for
years, in a study, the cells belonging to the pregnancy 27 years ago were
detected.
The free genetic material of the fetus in maternal blood
can be in the form of DNA and RNA. It is thought that the main source of DNA is
the apoptosis of sitsityotrophoblasts in the placenta. Since free DNA
originates from the placenta, placental mosaicism and fetoplacental
incompatibilities can rarely cause false positive and false negative results.
Similarly, the diagnosis can be misleading in mosaic trisomy situations.
Although free fetal DNA can rarely be detected as early
as 5 weeks (according to the last menstrual period) in pregnancy, it can
usually be detected after 9 weeks. The detection of free DNA in the mother's
blood before the fetal blood circulation begins suggests that this is due to
trophoblasts. The half-life of free fetal DNA in maternal blood was determined
to be short enough to be expressed in minutes. For this reason, it was reported
that it could not be detected in the mother's blood approximately 2 hours after
birth.
At the end of the first trimester, approximately 10-15%
of total DNA in maternal plasma consists of fetal DNA, while this rate
increases and reaches 50% in the last months of pregnancy. It was found that
the fetal DNA concentration decreased as the mother's weight increased. Lo et
al. found that fetal free DNA constitutes 2-6% of free DNA in maternal blood.
The free RNA half-life was also found to be short enough to be measured in
minutes, but was reported to be detected at a more stable concentration over
the months of gestation.
Free
fetal DNA test for prenatal screening:
- Free fetal DNA should not be used as a routine
screening test. It can be used as an additional screening test recommended by
informing the family.
- cffDNA is a high accuracy alternative to screening
tests used today, but it is too expensive to be routinely applied, so screening
tests (double, triple, quadruple test) should be continued today.
- It should not be recommended to low risk group and
multiple pregnancies because there is not enough evaluation in these groups.
- If the test result is negative, it does not guarantee
that the baby is completely normal.
- In case of positive test results, genetic counseling
and invasive tests such as amniocentesis and CVS should be recommended for
definitive diagnosis.
- The test only gives information about common trisomies
(21, 13, 18). It cannot give information about other genetic diseases. This
issue should also be mentioned while informing the family.
- Before the free fetal DNA test, family history should
be taken in terms of genetic diseases so that other tests can be applied in
terms of genetic diseases.
- If a baby's structural anomaly is detected on
ultrasonography, invasive tests such as amniocentesis should be recommended
because cffDNA can only detect trisomies.
- Free fetal DNA test cannot give accurate and precise
results like invasive tests such as amniocentesis. In this respect, these tests
have not been replaced, but it is an alternative option.
- It can be suggested as a screening test to the high
risk group in the indications listed below. It will identify babies with Down
syndrome with 98-99% sensitivity (with 0.5% false positivity rate).
When
free fetal DNA test is recommended:
- Pregnancies where the mother's age will be 35 or over
at birth
- Monitoring fetal "anoploidy" findings on
ultrasonography
- Pregnancy history with trisomy
- High risk detected in double, triple and quad screening
tests
- Balanced robertsonian translocation with increased risk
of trisomy 21 or 13 in parents
Areas
where free fetal DNA can be used outside of trisomies:
- It can be used to determine the sex of the baby in the
early weeks of pregnancy in the presence of X-linked miscarriages in the
mother. With other methods, sex will be detected later in the week and there
will be a low risk in invasive methods. Duchenne muscular dystrophy and
hemophilia are the most common x-linked diseases. These diseases are evident in
male babies because there is only one X chromosome and there is no second X
chromosome to compensate the diseased X chromosome.
- CffDNA analysis can be performed for genetic study in
families at risk for hereditary diseases.
- It can be used for Rh factor scanning. Due to the high
cost, it is not routinely used for this purpose, but it can be used in the case
of immunization.
- Some studies have reported that cffDNA was detected at
a higher concentration in pregnancies that will develop preeclampsia. It has
been reported that it can be used to predict preeclampsia.
- The determination of the gender of the baby in the very
early weeks of pregnancy (4th-5th weeks) with this method may cause
abortion-abortion procedures by choosing gender. In this respect, ethical
debates have occurred.
False
negativity situations:
- Not enough DNA material can be obtained
- The amount of cffDNA can differ between individuals and
is not found in sufficient quantity.
False
positivity states:
- Contamination
- Undetected vanishing-twin syndrome
- Placental mosaicism
- Maternal mosaicism
Approximately 2-5% of the blood samples taken for the
test cannot be obtained. In this case, blood should be collected again and sent
for analysis. This situation should be explained from the very beginning when
informing the patient.
It is considered that free fetal DNA analysis in maternal
blood will expand in the following years and its sensitivity will increase. It
will likely replace invasive tests completely and will be used for diagnostic
purposes, not screening. In the following years, widespread use of
high-sensitivity screening tests will provide significant benefits, because
today's prenatal screening tests do not detect approximately 15-20% of babies
with down syndrome (since the screening test results are normal). In addition,
1 out of every 20 pregnant women undergoing amniocentesis due to high risk is
found to have a baby with down syndrome, and 19 pregnant women are exposed to
unnecessary invasive procedures.
- FETAL NUCHAL THICKNESS MEASUREMENT
- NUCHAL TRANSLUCENCY SCAN TEST
- TRIPLE SCAN TEST
- WHAT IS QUADRUPLE BLOOD SCREENİNG TEST?
- INTEGRATED TEST
CHORİONİC VİLLUS SAMPLİNG (CVS)
FREE FETAL DNA ANALYSIS (cffDNA)
nuchal thickness
QUADRUPLE BLOOD SCREENİNG TEST
TRIPLE SCAN TEST
