Prenatal diagnosis
What is prenatal diagnosis?
Prenatal diagnostic testing is used to find out if a pregnancy has a particular inherited or genetic condition.
Diagnostic testing is different to prenatal screening tests.
- Screening tests during pregnancy will give an estimate of the chance that a pregnancy has a condition.
- Diagnostic testing will give a definite yes or no answer.
To perform diagnostic testing, a sample of the placenta or the amniotic fluid is needed. These samples are collected via invasive procedures called chorionic villus sampling (CVS) or amniocentesis (amnio). These procedures are associated with a small risk of miscarriage. Therefore, diagnostic testing is usually only offered when there is a clinical need.
The choice between a CVS or amniocentesis procedure can be discussed with your healthcare provider. Sometimes, an ultrasound during pregnancy can guide decision making regarding procedures.
When is prenatal diagnosis offered?
Testing is offered when:
- a woman receives a high chance result from a screening test (eg. non-invasive prenatal testing or combined first trimester screening)
- there are abnormal findings on an ultrasound
- there is a family history of a genetic condition
- the parents are both carriers of the same recessive genetic condition (eg. cystic fibrosis)
How is prenatal diagnosis performed?
There are two steps to prenatal diagnosis.
- The first is a procedure to collect the sample.
- The second step is testing the sample in a laboratory.
Procedures
Diagnostic testing is done on samples of the placenta or amniotic fluid. These are collected by:
- Chorionic villus sampling (CVS)– performed from 11 weeks
- Amniocentesis – performed from 15 weeks
Laboratory testing
There are a few different tests that can be done on the samples of placenta or amniotic fluid.
- FISH – looks to see if the cells have the correct number of chromosomes (for chromosomes X, Y, 21, 18 and 13)
- Chromosome microarray (CMA) – this test can identify extra or missing DNA in the chromosomes
- Single gene – looks to see if there is a genetic change in a particular gene that can cause a specific condition
How is prenatal diagnosis arranged?
Diagnostic procedures and testing of CVS or amniotic fluid samples is arranged through your doctor. Depending on the test, results take between 8 and 14 days. There is usually an out-of-pocket cost for diagnostic testing.