How Does Non-Invasive Prenatal Screening (NIPS or NIPT) Work?
What is NIPS?
Non-invasive prenatal screening (NIPS), also known as non-invasive prenatal testing (NIPT), is a prenatal genetic screen that can be done as early as 9 weeks gestation in a pregnant person of any age. NIPS first became commercially available in the US in 2011 with the launch of Sequenom’s MaterniT21 test for high-risk pregnancies, and has since become an increasingly common tool for prenatal genetic screening.
How does non-invasive prenatal screening work?
NIPS is called “noninvasive” prenatal screening because testing is performed on a blood sample rather than on direct prenatal samples such as amniotic fluid or placental tissue. This means an invasive procedure such as amniocentesis or chorionic villus sampling is NOT required to obtain the sample, and taking the test is safe for both the pregnant person and the baby.
During pregnancy, a person’s blood contains a mix of their own DNA as well as DNA from the pregnancy or placenta. NIPS analyzes small fragments of DNA in the blood called cell-free DNA (cfDNA). Unlike most DNA, which is found inside a cell’s nucleus, cfDNA fragments are free-floating and not within cells. Because the placenta and the fetus both grow from the same group of cells (embryo), placental DNA is usually identical to fetal DNA, so placental cfDNA can be analyzed via NIPS to screen for chromosomal conditions in the baby.
The exact method of genetic analysis for NIPS testing varies among labs, however, one common approach is whole-genome next-generation sequencing (NGS), also known as massively parallel shotgun sequencing (MPSS or MPS). With NGS/MPS, advanced technology simultaneously sequences (i.e., reads the code of) millions of short DNA segments and then compares the DNA fragments to a reference genome. Imagine taking puzzle pieces - some of which are individual pieces, others are 3 or 4 already assembled pieces - and comparing these pieces to a complete puzzle. You want to match the puzzle pieces to the final product to see where each piece goes and ensure that each and every puzzle piece is present and accounted for.
Extra or missing chromosomes can be detected by comparing the number of DNA pieces that map to a certain chromosome compared to the number of pieces mapping to other chromosomes. If more DNA pieces than expected map to a specific chromosome, this can indicate an increased risk that the baby has an extra copy of that chromosome (trisomy). For example, if NIPS detects a higher amount of DNA mapping to chromosome 18 compared to other chromosomes, this could indicate an increased risk for trisomy 18.
- Authored by Kelly Miller, MS, LCGC | Genetic Counselor
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