Non-invasive prenatal testing will be rolled out in the NHS next year. But how does it work? And what are the benefits?
Non-invasive prenatal testing (NIPT) involves examining fetal DNA by taking a sample of blood from the mother-to-be. Starting next year, the health service will provide this testing to women who are deemed to be at high risk of having an affected pregnancy, which means their unborn child may be at risk of developing a serious health condition.
The need for a new test
Traditional prenatal genetic testing techniques involved obtaining a sample of fetal cells through invasive methods such as chorionic villus sampling (CVS) or amniocentesis, both of which carry a risk of miscarriage. However, advancements in prenatal genetic testing have led to the development of non-invasive methods such as analyzing fetal DNA present in the mother’s blood.
These methods can provide accurate genetic information without posing a risk to the fetus. While other prenatal tests such as ultrasounds and hormone level analysis can indicate the likelihood of certain conditions, they do not provide a definitive diagnosis.
How does NIPT work?
Non-invasive prenatal testing (NIPT) is a method of examining the DNA fragments found in the mother’s blood, which includes cell-free DNA (cfDNA). While most of these DNA fragments will come from the mother, some of them will come from the placenta, carrying the fetus’s DNA.
Using this technique, it is possible to isolate and analyze the cfDNA fragments to detect a range of abnormalities. By identifying which chromosome each fragment comes from and analyzing the proportions of fragments, NIPT can determine if they come from a person with the standard 46 chromosomes or not. NIPT can also test the fragments for specific genetic markers to provide further insights.
What can it detect?
The test is primarily used to detect aneuploidies – where an abnormal number of chromosomes is present in each cell – specifically the trisomies that cause Down’s, Edwards’ and Patau’s syndromes.
As fetal sex is determined by chromosomes, accurate sex determination is also possible from nine weeks gestational age – much earlier than by ultrasound. Sex chromosome aneuploidies such as Turner syndrome (monosomy X) and Kleinfelter Syndrome (XXY) are also detectable.
The technique has also now been adapted to some rare genetic diseases that are caused by single gene variation, including cystic fibrosis and Apert syndrome. In these instances the result is definitive and does not need to be confirmed by invasive tests. This is referred to as NIPD – non-invasive prenatal diagnosis.
It has been proven possible to sequence the whole fetal genome from cfDNA, but it is currently too time consuming and costly to do routinely.
Testing in the NHS
In 2018, NIPT will be introduced for the screening of trisomies. The so-called ‘combined test’, which comprises an ultrasound scan and blood test for the mother’s hormones, will continue to be the first stage test, with women whose results show a high risk of an affected pregnancy – those who would currently be offered an invasive test – offered NIPT. Only if NIPT returns a positive result will they be offered an amniocentesis for a definitive diagnosis.
The Department of Heath estimates that this will reduce the number of invasive tests from approximately 10,000 per year to around 1,400, with just three miscarriages as a result.
NIPT is also available on the NHS for sex determination where the parents are known carriers of serious sex-linked genetic conditions, such as Duchenne Muscular Dystrophy, which disproportionally affects boys.
