Noninvasive Prenatal Genetic Testing for Down Syndrome

Down syndrome is a condition that develops as a result of an abnormality in the number of chromosomes an individual inherits. More specifically, an extra copy of the twenty-first chromosome causes both physical and mental developmental delays affected individuals.

It is typically associated with physical growth delays, characteristic facial features, and mild to moderate intellectual disability. The average IQ of a young adult with Down syndrome is 50, equivalent to the mental ability of an 8 or 9-year-old child, but this can vary.

Expecting parents are now able to learn more about their pregnancy through prenatal screening.

Guidelines recommend screening for Down syndrome to be offered to all pregnant women, regardless of age.A number of tests are used, with varying levels of accuracy. They are typically used in combination to increase the detection rate.None can be definitive, thus if screening is positive, either amniocentesis or chorionic villus sampling is required to confirm the diagnosis.Both of these tests are invasive .

Ultrasound imaging can be used to screen for Down syndrome. Findings that indicate increased risk when seen at 14 to 24 weeks of gestation include a small or no nasal bone, large ventricles, nuchal fold thickness, and an abnormal right subclavian artery. But the results are not that confirmed.

Noninvasive prenatal screening 

The discovery of fetal DNA in maternal blood combined with a new and very-high output sequencing method – NGS (Next Generation Sequencing), has opened numerous perspectives in the field of fetal medicine. One of the main expected applications is non-invasive Down’s syndrome screening (trisomy 21 – T21).

Noninvasive prenatal screening allows parents to understand the risks of having a child with a chromosomal abnormality and can allow them to make informed decisions regarding pregnancy care and the health of their child after birth.

How Does Noninvasive Prenatal Testing Screen for Down Syndrome?

Noninvasive prenatal testing requires a simple blood test from the mother.. During pregnancy, DNA from the placenta circulates in the mother’s bloodstream; this material is referred to as cell-free DNA. According to the American College of Obstetricians and Gynecologists, after gestational week 10, approximately 3-13% of all cell-free material circulating in the woman’s bloodstream is thought to originate from the placenta.

The DNA found in the placenta is identical to DNA belonging to the fetus. Scientists are able to isolate this placental DNA from the mother’s blood and analyze it for chromosomal abnormalities such as Down syndrome.

Testing of the mother’s blood for fetal DNA is being studied and appears promising in the first trimester. The International Society for Prenatal Diagnosis considers it a reasonable screening option for those women whose pregnancies are at a high risk for trisomy 21. Accuracy has been reported at 98.6% in the first trimester of pregnancy. Confirmatory testing by invasive techniques (amniocentesis, CVS) is still required to confirm the screening result.

How Does Noninvasive Prenatal Testing Differ From Other Prenatal Tests?

Noninvasive prenatal testing can be performed as early as week 10 in pregnancy, whereas other prenatal testing procedures must be performed later on, when the fetus is more developed. This procedure is used to analyze the risk that a developing fetus may have a chromosomal abnormality before the need for more invasive diagnostic procedures occurs.

Interpretation of results :

Results from these tests can be used to determine whether or not couples should consider diagnostic testing.

There are three possible results from NIPT for Down syndrome:

Positive: Predicted to be affected by Down syndrome. An invasive test should be offered to confirm the result.

Negative: Highly unlikely to be affected by Down syndrome.

Inconclusive: Inconclusive results happen in up to 4% of cases. This is usually because the proportion of fetal DNA present in the sample is not high enough to give an accurate result. NIPT may be repeated with the hope that the cffDNA levels will have increased due to the increased gestation.

Results usually take 10 days to two weeks to come back from the laboratory.

NIPT detects around 98% of all babies with Down, Edwards and Patau syndromes.

Advantages:

It minimizes the time

Easiest and fastest non-invasive way.

The patients who are scared of invasive procedures gets relaxation.

Additionally, the use of non-invasive prenatal testing can significantly reduce the need for invasive prenatal procedures, which are associated with a one percent chance of miscarriage and significant risk of serious infection  (roughly one in 1,000).

Who is a Candidate for Noninvasive Prenatal Testing?

During your preconception and/or your first prenatal appointment, your doctor may recommend noninvasive prenatal  invasive:

You are of maternal age 35 years or older

You have previously given birth to a childrecommendc abnormality

You or your partner have a family history of a chromosomal abnormality

You have received an abnormal result from a first-trimester ultrasound exam

Where Can I Learn More About Noninvasive Prenatal Testing?

If you are planning a pregnancy or if you are currently pregnant and would like to learn more about the health of your child, consult a genetic counselor about noninvasive prenatal testing. Your doctor or genetic counselor can help educate you on the types of prenatal tests available as well as which tests may be the most appropriate for you and your child.

Other advances:

NIPT will also provide results for three rarer chromosomal conditions:

Edwards syndrome (trisomy 18)

An extra copy of chromosome 18 is present.

NIPT detects elevated levels of chromosome 18 DNA sequences, which would suggest the baby has Edwards syndrome.

Detection of Edwards syndrome is as accurate as for Down syndrome (>98%).

Patau syndrome (trisomy 13)

An extra copy of chromosome 13 is present.

NIPT detects elevated levels of chromosome 13 DNA sequences, which would suggest the baby has Patau syndrome.

Detection of Patau syndrome is as accurate as for Down syndrome.

Turner syndrome

Only one copy of the X chromosome is present in a female baby.

NIPT detects a reduced level of X chromosome sequences and no Y chromosome sequences, indicating that the baby is likely to have Turner syndrome.

Detection of Turner syndrome is not as accurate as for the other aneuploidies.

Conclusion :

Thus , its an effective technique with low risks and it should be practiced in developed as well as under developed countries where disease byrden is much more.