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It is now possible to use NIPT to detect Down syndrome and three of the other common aneuploidies: Edwards syndrome, Patau syndrome and Turner syndrome. Further research and evaluation is necessary before these tests are introduced into the NHS, although they are already available in the private sector. This guide will cover the key facts that patients and health professionals need to know about NIPT for Down syndrome. People normally have 23 pairs of chromosomes in each cell (see the karyotype, below left). Each chromosome is made of a coil of DNA which contains our genes. NIPT for Down syndrome works by analysing the DNA fragments present in the maternal plasma during pregnancy. Although both tests involve a sample of maternal blood, NIPT analyses the cell-free DNA in the mother’s blood, whereas the combined and quadruple test analyses the mother’s hormone levels. With more than 98% accuracy, NIPT is more accurate than the first trimester combined test or second trimester quadruple test for estimating the chance or the risk that the baby has Down syndrome. When used early in pregnancy, cell-free DNA from a ‘vanishing twin’ may be present, as the placenta continues to shed fetal DNA after embryonic demise. The cell-free fetal DNA comes from the placenta, and we know that sometimes there are ‘cell lines’ that grow in the placenta but not the baby. Very occasionally, because we are testing all the cell-free DNA in the mother’s blood (this includes the mother’s and the baby’s DNA), we may detect a problem that is present in the mother but not the baby.
It is possible that in the future NIPT could replace current screening tests or diagnostic tests.


Blood samples are sent for testing in the US and results are usually returned within two weeks.
There are several issues women should be encouraged to consider before deciding whether to have NIPT. NIPT detects elevated levels of chromosome 18 DNA sequences, which would suggest the baby has Edwards syndrome. NIPT detects elevated levels of chromosome 13 DNA sequences, which would suggest the baby has Patau syndrome.
NIPT detects a reduced level of X chromosome sequences and no Y chromosome sequences, indicating that the baby is likely to have Turner syndrome. For information on other ways in which NIPT is being used, see our other guides for patients and healthcare professionals. This is usually because the proportion of fetal DNA present in the sample is not high enough to give an accurate result. This is called ‘confined placental mosaicism’ causing a false positive result, which reflects an abnormal cell line that is only present in the placenta but not present in the baby.
This can cause a false negative result if an abnormal cell line is only present in the baby but not in the placenta. However, research is being done to evaluate the technology and assess how it may be introduced.


However, it is currently being evaluated as an additional step in the existing screening pathway. Most of this cell-free DNA comes from the mother (cell-free maternal DNA), but around 10%-20% of it comes from the unborn baby (cell-free fetal DNA). This allows very accurate prediction of pregnancies where the fetus is likely to have Down syndrome. The cffDNA is cleared from the maternal circulation within the first hour after birth, and therefore we know that it is specific to the woman’s current pregnancy.
NIPT may be repeated with the hope that the cffDNA levels will have increased due to the increased gestation.
As described before, there are false positive results and it is likely that invasive testing will be required to confirm a positive NIPT result for the foreseeable future. However, it is important to remember that NIPT for Down syndrome analyses both the baby’s and the mother’s cell-free DNA. In order for you to see this page as it is meant to appear, we ask that you please re-enable your Javascript!



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