The combined test in the 1st trimester (that includes the measurement of the nuchal translucency, assessment of the nasal bone, the flow pattern through the tricuspid valve, the flow pattern in ductus venosus and the maternal biochemistry results) detects Down syndrome in about 93- 95%. That means that the rest of the cases cannot be detected. Moreover, approximately 5% of women tested will become unjustifiably worried since the test will show an increased probability of Down syndrome without the baby been affected (false positive result). Such are the limitations of this, admittedly, very good test that is in our disposal.
What is the Non- invasive Prenatal Test (ΝIPT)?
Non- invasive prenatal test is the ‘next big thing’ in antenatal diagnosis. According to some, this is a true revolution in the field of Fetal Medicine.
The test is done by simply taking a blood sample from the mother, anytime after the 9th- 10th gestational week. The test is based on the fact that a small amount of the baby’s genetic material (cell- free fetal DNA – cffDNA), coming from the placenta, is present in the mother’s blood circulation. This genetic material can be isolated and tested using highly sophisticated techniques for chromosomal abnormalities, namely Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome). The test is not diagnostic, but has an accuracy of >99% in correctly identifying babies with Down syndrome. The false positive results are only 0,1% of the cases.
How is the Non- invasive Prenatal Test (ΝIPT) done?
An ultrasound scan needs to be done before the test, to document the fetal heartbeat and exclude any structural abnormalities. The test is done by simply taking a blood sample from the mother. Subsequently, the blood sample is sent abroad to a specialised laboratory in order to analyse and interpret the results.
What can the Non- invasive Prenatal Test (NIPT) detect?
Current data suggests that ΝΙΡΤ can detect the commonest chromosomal abnormalities, that is Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18) and Patau syndrome (Trisomy 13) and can also detect fetal sex with great accuracy. The detection rates for the above mentioned syndromes are extremely high for Down syndrome and is estimated to be >99%. For Edwards syndrome the detection rate is 97% and for Patau syndrome is 92%. Detection rates deteriorate by quite some for other chromosomal abnormalities, such as sex chromosomes abnormalities, triploidy, mosaicism and transpositions. Upon fetal indications, testing for some microdeletion syndromes like DiGeorge, Angelman, Prader-Willi, Cri-du-chat and 1p36deletion is feasible, however the accuracy of the test in such cases has not been validated by large studies. So, for the time being, there is no official indication to use NIPT for these cases.
What are the Non- invasive Prenatal Test (NIPT) limitations?
This method is not diagnostic, which means that it cannot replace the invasive testing for chromosomal abnormalities. (CVS, amniocentesis). It can detect babies with Down syndrome in >99% but for trisomies 18 and 13, detection rates are somewhat lower.
The test is not always feasible. In about 4- 5% of the cases a result cannot be reported. This is due to technical reasons and has to do with the fact that the fetal fraction of the fetal DNA in the mother’s blood is not high enough to produce a meaningful result (for example, there is a higher probability of failure in obese women). In that case, the NIPT test needs to be repeated or we have to choose an alternative method.
For the time being, the highly accurate test options are limited to trisomies 21, 18 and 13. Detecting other, less common, chromosomal or genetic diseases lacks accuracy.
What are the Non- invasive Prenatal Test (NIPT) indications?
The fact that this test carries no risk for miscarriage, since it is only a blood test, renders the test extremely useful in cases where:
- There is intermediate probability of chromosomal abnormalities (chance from 1/ 300 to 1/ 1500).
- Parents wish to have an extra test for reassurance purposes even if the probability of chromosomal abnormalities is low (less than 1/ 1500).
- Parents wish to avoid the miscarriage risk after the invasive procedure (especially when there is a possibly high risk of miscarriage or if this is an IVF pregnancy).
- There is a history of chromosomal abnormalities in a previous pregnancy.
When is the Non- invasive Prenatal Test (NIPT) not indicated?
The test is not indicated in cases where:
- The probability for Down syndrome, as calculated by the nuchal translucency scan, is significantly increased (more than 1/ 300) or there is some structural abnormality discovered during the ultrasound scan. In this case, one of the diagnostic invasives procedures (CVS or amniocentesis) is indicated.
- Parents want to know with certainty if the baby has or not Down syndrome.
- Testing for other, less common, chromosomal or genetic diseases is needed.
When will the results be back?
Results are available in approximately 10- 15 days from the day the blood was collected. As soon as we have the results back, we will inform you immediately.
What do we expect from the results to show?
If the test shows that there is an increased probability that the baby is affected by trisomy 21, 18 or 13, there is an absolute need for it to be confirmed, as there is a small chance of false positive results (approximately 0.1%). In that case, confirmation needs to be done with one of the invasive procedures that are diagnostic (CVS or amniocentesis).
If the test shows low probability (less than 1/ 10000) then it is highly unlikely that the baby is affected byt trisomy 21, 18 or 13. However, it has to be said that a negative result by no means guarantees that the baby is chromosomally normal, just reduces the probability by a great factor.
What happens after the test?
Depending on the stage of pregnancy, and if it has not been done already, the nuchal translucency scan can be done in order to examine the baby for any structural abnormalities. Later on, the anomaly scan needs to be done and the growth/ Dopplers scan at 30- 32 weeks.