Skip to main content
Public beta This website is in public beta – please give your feedback.

Clinical applications

In non-invasive prenatal testing (NIPT), a maternal blood sample is used to screen for common fetal aneuploidies. Aneuploidy is the presence of an abnormal number of chromosomes in a cell. The three types of aneuploidy currently screened for in pregnancy are Patau syndrome (trisomy 13), Edward syndrome (trisomy 18) and Down syndrome (trisomy 21). NIPT is also available privately, where a wider range of aneuploidies may be screened for.

How does it work?

NIPT is performed using a sample of maternal blood. This contains a mixture of maternal DNA and placental DNA, known as cell-free DNA (cfDNA). In most cases, the placental DNA will be representative of the fetal genome.

The cfDNA can be isolated, and it is possible to identify which chromosome each isolated fragment of DNA comes from. Techniques that can be used for this include next-generation sequencing and microarray . By looking at the proportions of fragments derived from each chromosome, it is possible to determine whether there is an excess of fragments from any particular chromosome.

Advantages and limitations of NIPT

Advantages

  • There is no need to take a sample from inside the uterus (in contrast with invasive prenatal diagnosis using amniotic fluid or chorionic villus sampling). This removes the small risk of miscarriage associated with invasive procedures.
  • It is available for twin pregnancies.

Limitations

  • It is not designed to detect fetal genetic conditions other than the targeted aneuploidies (some of these, for example cystic fibrosis, may instead be tested for using non-invasive prenatal diagnosis).
  • Currently, NIPT is only available on the NHS to women who have had a screening blood test result (NHS combined or quadruple test) that suggests there is a high chance of fetal trisomy.
  • NIPT is an advanced screening test, not a diagnostic test. An invasive prenatal test is required to confirm all high-chance NIPT results. This is because, rarely, the placental cfDNA will not be representative of the fetal genome owing to confined placental mosaicism (CPM). In CPM, the aneuploid cells are present in the placenta but not in the fetus. This can lead to a false positive NIPT result.
  • It is not available in triplet or higher multiple pregnancies.
  • It is not available to pregnant women who have features that may interfere with testing, for example Down syndrome, a balanced chromosomal translocation, or cancer (as the tumour may release cfDNA).
  • It is not available to women who have had an organ transplant or a recent (less than four months ago) blood transfusion.

Practicalities

  • A quantity of 20ml of pregnant patient blood is required in cell-stabilising blood tubes (Streck or PAXgene).
  • The minimum gestation is from 11 weeks.
  • Testing must be arranged in advance through your local antenatal clinic or fetal medicine unit.
  • Results are normally available within 10 days of blood draw.

Tagged: Technologies

↑ Back to top
  • Last reviewed: 14/07/2022
  • Next review due: 14/07/2024
  • Authors: Dr Julia van Campen
  • Reviewers: Professor Barbara Jennings, Dr Siobhan Simpson