What Is Diagnostic Testing?

A diagnostic test is different from a screening test in that it can confirm whether or not the baby has a chromosomal abnormality, like Down syndrome, or another genetic disorder.

A diagnostic test is generally offered to a pregnant person who has had a baby with a chromosomal abnormality before or has received an ‘increased risk’ result from her first trimester combined screening or maternal serum screening. Occasionally a diagnostic test is offered to screen for inherited conditions like cystic fibrosis.

Like other prenatal screening, diagnostic testing is your choice. For every 200 pregnant people who have diagnostic tests, one or two will have a miscarriage. Take time to discuss your options with your midwife or doctor before making a decision.

There are two types of prenatal diagnostic tests: chorionic villus sampling (CVS) and amniocentesis.

The CVS test is carried out before 14 weeks and involves a small sample of the developing placenta/whenua being taken.

An amniocentesis procedure is generally carried out from 15 weeks onwards, where a small amniotic fluid sample is collected from around the pēpi.

To collect a sample for the diagnostic test, an ultrasound will be performed to determine the baby’s position. A fine needle will then be inserted through the skin on your belly and into the placenta or fluid to withdraw the sample. The ultrasound will also help to guide the positioning of the needle.

Once the sample is collected, chromosomal analysis is carried out by a laboratory and results are generally available from your LMC 10-14 days following the procedure.

If your baby is found to have a genetic condition, your midwife, doctor or specialist will talk through the results with you, what this means for your child, and provide you with information and support. You will be given the opportunity to choose whether to continue or terminate the pregnancy. This can be an extremely difficult decision to make, but however you choose to proceed, social work or other support services will be available to you.