Screening for genetic disorders isn’t new, but the way you can be screened for them is. NIPT - Non-invasive prenatal testing carries less risk to pregnant women then previous screening options.
Non-invasive prenatal testing (NIPT) involves a blood screening which can analyse your baby’s DNA to identify the risk for genetic disorders such as Down’s syndrome. Previously other screening tests were available however these were invasive tests which carried the risk of miscarriage. NIPT tests work by analysing cell-free maternal and fetal DNA (cffDNA) from the mother’s blood. This is a non-invasive test as all it requires is for mum-to-be to have a blood test.
What conditions can NIPT identify?
Although most commonly associated with testing for Down’s syndrome (trisomy 21), NIPT can also detect the presence of other disorders such as Edward’s (trisomy 18) and Patau’s syndrome (trisomy 13). Evidence suggests that NIPT is the most accurate screening test for identifying these conditions.
How does NIPT work?
NIPT involves a quick draw of blood from mum-to-be, so all you’ll need to do is offer up your arm at the doctor’s which is a safe procedure for you and your baby. Having given blood, your sample will be sent off by the surgery or hospital to review your blood and baby’s DNA for any abnormalities.
You can get the test from 10 weeks pregnant as before this it is difficult to identify the cell-free fetal DNA (cffDNA) from the blood. Even after this time it can be that there isn’t enough cffDNA to come to a conclusive test, and in most cases you’ll then have to go for a second test. This happens to about 3% of women.
Results can take from 10 days to 2 weeks to come through. It’s important to remember that your results won’t show you a definite yes/no for Down’s syndrome or other conditions, the results instead will show you the risk level of your baby being affected. If the result is ‘low risk’ it is unlikely that they will be affected, if the result is ‘high risk’ it is likely you will be offered a CVS or amniocentesis to determine a more definite diagnosis.
How does NIPT detect Down’s syndrome?
People usually have 23 pairs of chromosomes in each cell, which is made of DNA which contains your genes. These cells give instructions to our body to tell it how to work. Down’s syndrome occurs when a person has an extra copy of chromosome 21 and this is what causes the physical and mental characteristics of a person with Down’s Syndrome. Identifying the extra copy of chromosome 21 is possible with NIPT.
The test analyses the DNA in the maternal plasma during pregnancy, also known as cell-free DNA. Most of this DNA comes from the mother, but 10% - 20% of it comes from the unborn baby and this is known as cell-free fetal DNA (cffDNA). The NIPT test analyses and counts the DNA sequences in the mother’s blood, if there are more sequences that map to chromosome 21 than expected it can be an indication that the baby has Down’s Syndrome.
This cell-free fetal DNA comes from the placenta and is detectable from around 4-5 weeks, and has the required levels needed to identify the syndrome in NIPT at 10 weeks. The cffDNA is cleared from the mother within the first hour after birth, which means the results of the test are specific to a woman’s current pregnancy.
How is NIPT different from the combined/quadruple test?
The quadruple test screens for Down’s syndrome and spina bifida, and is available between 14 weeks and 2 days and 22 weeks and 6 days of your pregnancy (though ideally before 20 weeks). This test measures the amount of four hormones in the blood - AFP, beta HCG, inhibin-A and oestriol. The levels of these are combined with details of your age, weight and how far into the pregnancy you are to analyse the risk of your baby having Down’s syndrome.
Although both these tests involve a blood test, NIPT analyse the cell-free DNA rather than the above hormones. NIPT has more than 98% accuracy, and so more accurate than the combined and quadruple tests for determining the risk of Down’s Syndrome in your baby.
Why are the results only 98% accurate?
False positives: False positives may occur if the test is done too early in pregnancy, as the cell-free DNA from fetal DNA can be shed after embryonic demise, also known as a ‘vanishing twin’. If this happens a scan is needed to look for multiple pregnancies or an empty sac before the test happens.
Another reason for a false positive may occur if the tested cell-free DNA in the mother’s blood shows an anomaly and it’s not from the baby’s blood.
Finally the cffDNA may identify cell lines that are grown in the placenta but are not present in the baby. This is called confined placental mosaicism and can cause a false positive result as the cell is present in the placenta but not in the baby.
False negatives: This could occur if the proportion of fetal DNA in the blood is too low or technical issues. It can also happen as there are ‘cell lines’ that grow in the placenta but not the baby which can cause a false negative result if an abnormal cell line is only present in the baby but not in the placenta.
Who is eligible and how much does it cost?
The non-invasive prenatal test (NIPT) will be rolled out by the NHS from 2018.
You can however get private tests done and prices are between £400 - £900.