NIPT is a DNA test on maternal blood to screen pregnancies for the most common fetal chromosome anomalies: trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome.
NIPT for Down syndrome works by analysing the DNA fragments present in the maternal plasma during pregnancy. This is known as cell-free DNA. Most of this cell-free DNA comes from the mother (cell-free maternal DNA), but around 10%-20% of it comes from the unborn baby (cell-free fetal DNA).
Each chromosome has sequences of DNA that are specific to that particular chromosome. Therefore, by analysing and counting all the DNA sequences that link or map to each individual chromosome, the total amount of chromosome 21 in the mother’s blood can be compared with the amount of the other chromosomes. If the baby has Down syndrome, there will be slightly more sequences that map to chromosome 21 than expected, indicating that there is more chromosome 21 present than normal. This allows very accurate prediction of pregnancies where the fetus is likely to have Down syndrome.
Cell-free fetal DNA (cffDNA) comes from the placenta. It is first detectable from about 4-5 weeks’ gestation and reaches the required level needed to test for Down syndrome by 10 weeks’ gestation in most pregnancies. The cffDNA is cleared from the maternal circulation within the first hour after birth, and therefore we know that it is specific to the woman’s current pregnancy. However, it is important to remember that NIPT for Down syndrome analyses both the baby’s and the mother’s cell-free DNA.