What is non-invasive prenatal test (NIPT) and how is it done?

It is the latest development in prenatal screening for chromosomal abnormalities.

The test is harmless to the fetus and is performed by simple blood sampling from the mother. In the mother's blood there is an amount of genetic material (DNA) that is free, and comes from cells that are dead and lysed (cell-free DNA, cfDNA). After 4 weeks of pregnancy a fraction of cfDNA is derived from the fetus (fetal cfDNA), and more precisely from the placenta. After 9 weeks of gestation, fetal cfDNA accounts for more than 4% (usually around 7-10%) of total cfDNA in the mother's blood. This DNA is isolated from the mother's blood and then checked for chromosomal abnormalities.

The purpose of the examination is to determine if the fetus is in low or high risk for a small number of severe conditions.

What is the detection rate does it have?

It detects 99.5% of fetuses with Trisomy 21 (Down syndrome), 97% for trisomy 18 and 92% for trisomy 13. It occurs in both single and twin pregnancies. In addition, it detects the fetus's sex very accurately.

What are the limitations?

The results of the test are not diagnostic but give us probabilities and therefore are not absolute. It has the potential to give us either a very low (<1/10000) or a very high risk (70-90%) of the fetus being affected. A "low risk" result greatly reduces the chance of the fetus having an abnormality in the chromosomes 21,13,18 but cannot guarantee a normal number of chromosomes or the birth of a healthy child.

A 2-5% of the samples will not work. Fetal DNA fragment plays an important role in the extraction of the results and this may be affected by the mother (eg smaller in obese women) or the fetus (in fetuses with trisomy 13 is 92%). So a mother doing this test does not mean that all the possible diseases that the fetus may be suffering from have been investigated. In contrast, both chorion villus sampling and amniocentesis can detect a much greater number of diseases and syndromes than non-invasive tests.

The non-invasive test does not give us information about rare chromosomal abnormalities or various genetic syndromes / diseases. If the fetus has anatomical abnormalities, increased nuchal translucency then CVS or amniocentesis is recommended. Furthermore, it does not provide information on congenital abnormalities, such as abnormalities of the heart or brain and spine, or fetal development. Therefore, it is suggested to carry out the scans in the first, second and third trimester.