What Is NIPT For Down Syndrome?
Non-invasive Prenatal Testing (NIPT) is a prenatal screening done by collecting a mother’s blood sample to identify the risk of the fetus being born with certain chromosomal disorders.
However, a screening like NIPT cannot determine whether the baby has a chromosomal disorder, only the likelihood of having that condition. But even though it can’t tell whether the baby has a genetic abnormality, it is highly accurate — 97 to 99 percent accuracy for three of the most common conditions. – Down syndrome, Edward’s syndrome, Patau syndrome.
The results of a NIPT screening can help determine whether to have a diagnostic test like Chorionic Villus Sampling (CVS) or amniocentesis. CVS and amniocentesis analyse a baby’s own genetic material, collected from the placenta or amniotic fluid, respectively, to tell with 100 percent certainty whether a baby has a chromosome abnormality. However, they are invasive, which means they slightly increase the chance of miscarriage.
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Here, your pregnancy scans and tests are done under the care of Dr Deepthi Jammi, an award-winning Obstetrician and Gynaecologist in Chennai.
How Do NIPTs Work?
NIPT works by analysing fragments of DNA from the baby, which are naturally present in the mother’s bloodstream. These fragments are called cell-free DNA (cfDNA).
Digital analysis of this cfDNA can identify babies with the following chromosomal abnormalities:
The results are reported as a probability. A low-risk test result would be less than 1 in 1,000 likelihood (or a 0.01 per cent chance that the baby has the abnormality), while a very high-risk result would be 1 in 2 likelihood (or a 50 per cent chance the baby has the abnormality).
As it is only a screening test, if the result shows a high likelihood of a chromosomal abnormality, it’s still necessary to carry out a diagnostic test, such as CVS or amniocentesis, to determine whether the baby has the abnormality.
NIPT results are usually returned within 15 days if the samples are sent overseas for analysis.
What Will NIPT Reveal?
The results of a NIPT will indicate the likelihood – or risk factor – that the baby has one of the abnormalities tested for and does not provide a “Yes/No” answer. If the baby is highly likely to have an abnormality, the obstetrician may suggest a diagnostic test, such as CVS or amniocentesis.
NIPTs only test for specific chromosomal abnormalities. They can’t detect major malformations such as spina bifida (defect in the baby’s spine), so the mother should still have an ultrasound at around 12 weeks to see how the baby develops.
The mother can choose to have a NIPT instead of a first trimester combined screening test, an ultrasound and a blood test. A combined screening test is not as accurate as a NIPT, which is claimed to have an accuracy rate higher than 99 percent.
Some patients choose to have a NIPT after another first-trimester screening test returns a high-risk result.
When Is The Test done?
How Is NIPT Done?
How Is NIPT Different From The Combined/Quadruple Test?
Are There Any Risks With Having An NIPT?
What Results Can We Get From NIPT For Down Syndrome?
There are three possible results from NIPT for Down syndrome:
Accuracy And Reliability Of NIPT
The NIPT is still a highly accurate screening test that gives a risk factor, not a diagnosis.
NIPT can be used as screening tests for low-risk and high-risk women. As the chance of genetic disorders increases with age, it’s expected that older women would benefit most from testing and may be able to avoid invasive tests.
There are several reasons why false-positive results happen; for example, if the test is done early in the pregnancy, it may detect DNA leftover from a vanishing twin that has been absorbed by the mother or by the surviving twin. In rare cases, maternal cell-free DNA may be mistaken for the baby’s, and the mother’s genetic problems may be attributed to the baby.
If the mother is overweight or obese, there may be too little baby DNA in her blood, which may cause a false-negative test result or an indeterminate result.
There is another possible reason for inaccurate NIPT results: cell-free fetal DNA comes from the placenta, and there are sometimes “cell lines” grow in the placenta but not in the baby. This may cause a false-negative test result if an abnormal cell line is present in the baby but not present in the placenta, or a false positive if present only in the placenta, not in the baby.
How Does NIPT Detect Down Syndrome?
People usually have 23 pairs of chromosomes in each cell (see the karyotype below at the left). Each chromosome is made of a coil of DNA that contains genes. These give instructions to the body to tell it how to work. Down syndrome occurs when a person has an extra copy of chromosome 21 (see the karyotype below, at the right). Having this extra genetic information is what causes the malformed physical and mental characteristics of people who have Down syndrome.
NIPT also checks for,
- Trisomy 18 – Edward’s syndrome – a condition that causes severe physical and mental abnormalities; it is the result of an extra chromosome, known as trisomy-18
- Trisomy 13 – Patau syndrome is a rare but severe condition where babies rarely survive for more than a few days; it results from an extra chromosome, trisomy-13.
Each chromosome has sequences of DNA that are specific to that particular chromosome. Therefore, by analysing and counting all the DNA sequences that link or map to each chromosome, the total amount of chromosome 21 in the mother’s blood can be compared with the amount of the other chromosomes. If the baby has Down syndrome, slightly more sequences will map to chromosome 21 than expected, indicating more chromosome 21 presence than normal. This allows very accurate prediction of pregnancies where the fetus is likely to have Down syndrome.
Cell-free fetal DNA (cffDNA) comes from the placenta. It is first detectable from about 4-5 weeks’ gestation and reaches the required level needed to test for Down syndrome by 10 weeks gestation in most pregnancies.
Frequently Asked Questions
A positive result suggests that the fetus has the genetic problem tested for. In this case, your doctor will discuss the results with a genetic counsellor. They can help you understand the problem, talk to you about your options, and help you decide what to do next.
Genetic amniocentesis is usually performed between weeks 15 and 20 of pregnancy. It is known that amniocentesis done before week 15 of pregnancy is associated with higher rates of complications.
There is a small risk associated with your baby during amniocentesis. This is approximately less than 1%. Other infrequent complications can be an injury to the baby or mother, infection, and preterm labour.
Amniocentesis gives a definitive result. It has a reliable rate of 98%. However, it can’t pick all genetic problems or other abnormalities
The test checks for fetal abnormalities (birth defects) such as Down syndrome, cystic fibrosis or spina bifida. Other genetic problems, or neural problems, and Rh incompatibility cannot be detected by amniocentesis. Talk to your doctor to be well informed before the procedure.