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Section 14 – What is a single nucleotide polymorphism (SNP) array test or a targeted (SNP) array test?

A single nucleotide polymorphism array (SNP) is a test that looks at all the chromosomes in the cells in the chorionic villus sampling (CVS) or amniotic fluid sample. The test looks for small changes in genetic material. 

An SNP test is offered when there are certain structural changes seen on an ultrasound scan.

A 'targeted' SNP array only looks at specific chromosomes and is offered for parents who are carriers of a chromosome rearrangement. A chromosomal rearrangement is a change in the structure of a chromosome, resulting in a change in the order or content of the genetic material. 

The midwife, genetic counsellor or hospital doctor (obstetrician) will discuss with you which test you are being offered. 

The SNP array test:

  • will not detect changes in single genes, such as cystic fibrosis (each chromosome contains thousands of genes).
  • may identify a chromosomal change that may not be related to the reason that you had the test performed (these changes may have implications for the future health of your baby or other members of your family), and
  • may show some chromosomal conditions that have an unknown effect on your baby.

This test looks for chromosomal changes in more detail than the QF-PCR test.

A 'targeted' SNP array test:

  • is performed when parents are carriers of a chromosome rearrangement
  • only looks at specific chromsomes
  • will not rule out changes to your baby's other chromosomes; and
  • will not show the sex of your baby

SNP array test and targeted SNP results 

These results are usually ready within about 14 calendar days.

Sometimes it takes up to 21 calendar days to complete these tests (this does not mean there is more chance of a structural or chromosomal anomaly with your baby).

Occasionally, in about 0.4% (1 in 250) of tests you will not get a result. If this happens, you may be offered another amniocentesis.

In some cases an SNP array test may give a result that is not clear. Your obstetrician will discuss this with you if this happens.

The SNP array test carried out when there are certain structural changes seen on an ultrasound scan will show the sex of your baby. If you do not want to know the sex of your baby, please tell your midwife or obstetrician.

The targeted SNP carried out for parents who are carriers of a chromosome rearrangement will not show the sex of your baby.