Non-invasive prenatal genetic testing during pregnancy
Non-invasive prenatal testing (NIPT) screens for the presence of specific chromosome disorders in your developing baby. The test analyses fragments of DNA in your plasma that have been released from both your cells, and placental cells.
NIPT requires a single blood test, which poses no threat to your unborn baby, and can be done as early as 10 weeks’ gestation. By analysing the proportions of DNA fragments derived from different chromosomes or chromosome regions, NIPT can screen for the presence or absence of specific chromosome disorders.
NIPT is more accurate than first trimester maternal serum screening and ultrasound in identifying pregnancies with or without these specific disorders.
About the test
During a pregnancy, cell-free DNA can be tested to give the most accurate screening approach in estimating the risk of a fetus having a common chromosome condition sometimes called a trisomy. This occurs when there are three copies of a particular chromosome instead of the expected two. The test screens for the following conditions:
Trisomy 21 is the most common trisomy at the time of birth. Also called Down syndrome, it is associated with moderate to severe intellectual disabilities and may also lead to digestive disease, congenital heart defects and other malformations.
Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome) are associated with a high rate of miscarriage. These babies are born with severe brain abnormalities and often have congenital heart defects as well as other birth defects. Most affected babies die before or soon after birth, and very few survive beyond the first year of life.
Sex chromosome conditions occur when there is a missing, extra, or incomplete copy of the X or Y chromosomes. The NIPT test with sex chromosome aneuploidy panel option can assess risk for XXX, XYY, XXY (Klinefelter syndrome), and a missing X chromosome in a girl (Turner syndrome).
In addition, NIPT can assess fetal sex. This is optional (no additional cost).
NIPT does not screen for non-chromosome disorders, familial mutations, malformations, fetal growth or fetal viability.
Risk
The test involves a simple blood test, so does not put the pregnancy at risk.
Accuracy
NIPT provides fewer false positive and false negative results than the NHS combined test for trisomy 21, 18 and 13.
It does not give a definitive genetic diagnosis for your unborn baby, which requires genetic analysis by amniocentesis or CVS testing.
Accuracy (T21, 18, 13) |
Sensitivity |
False positive rate |
Combined test |
82% |
1 in 26 |
NIPT |
>99% |
<1 in 1000 |
Who can have the test?
Women with pregnancies of at least 10 weeks’ gestational age. The test can be requested for any singleton or twin pregnancy, including those conceived naturally or by IVF using the patient’s own egg or a donor egg.
In twin pregnancies, sex chromosome (X and Y) analysis can determine fetal sex but not sex chromosome conditions. This test is not suitable in the case of a vanishing twin. The sex of twins will be reported as one result. If reported as male, one or both of the twins will be male. If reported as female, both twins will be female. The test does not assess risk for mosaicism, partial trisomies or translocations.
Results
The sample is sent securely to TDL laboratory in London, and results will usually be available within 5 working days of having the test. You will have a choice of how to receive the results. Many couples choose to receive an email first which they can open at a convenient time, followed up by a telephone call.
Repeat Samples
There needs to be enough fetal DNA in the maternal blood to be able to provide a result. If there is insufficient fetal DNA in the sample (which occurs in 2% of cases), another blood sample will be required. There is no extra charge.
If an assessment cannot be provided
On rare occasions, NIPT is unable to provide an assessment of the probability of specific chromosome disorders. This usually reflects the complex biology of genetics and pregnancy, and is not due to a failing in the laboratory.
If NIPT cannot provide a specific assessment, then alternatives will be discussed with you depending on your individual situation.
How much does it cost?
The cost of the initial consultation, taking and analysing the initial blood test, any additional blood tests, and discussing the results is £520. You will receive an invoice from TDL laboratories, usually after you have received the results.
Do I need a scan?
You will need to have had a dating ultrasound scan to check the viability and to check for multiple pregnancies. This can usually be arranged at the time of the test.
Non-invasive prenatal testing (NIPT) | The Doctors Laboratory (tdlpathology.com)
Richard Smith PhD MRCOG Consultant Obstetrician, Subspecialist in Fetal Medicine, Norfolk & Norwich University