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.
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 between 11-13 weeks
- Amniocentesis – performed after 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.