Genetic Screening and Diagnostic Testing in Your Pregnancy

There are several genetic screening and diagnostic tests that may be offered to you in pregnancy.

Your choice to complete genetic screening or diagnostic tests is a personal one. It is worthwhile considering what you may do with the information, in determining which if any options are right for you.

This online decision-aid can help you decide what genetic screening tools or diagnostic tests, if any, might be right for you and your pregnancy.

What is the difference between a screening tool and a diagnostic test?

A screening test estimates the risk of having a certain condition, it is non-invasive and generally has no or few physical side effects. It can result in false positive or negatives.

A diagnostic test, by comparison, is targeted testing that provides you with a definitive result as to the existence or absence of a certain condition. It may carry risks or side effects, and generally has high specificity.

Only a diagnostic test, such as an amniocentesis or chorionic villus sampling (CVS), can definitively tell you whether your baby has a genetic or chromosomal condition.

Given the risks of miscarriage associated with the diagnostic tests, many women and birthing people elect to have a screening test before deciding whether to complete diagnostic testing.

What choices do I have for genetic screening?

There are two primary options:

  • Combined first trimester screen

    This is a combined screening tool, using the first trimester ultrasound taken at 11 to 13 weeks (and particularly, the measurement of the back of your baby’s neck, “nuchal translucency”) and a blood test, taken at 10–14 weeks of pregnancy, to estimate the chance of your baby having Down Syndrome. It is accurate for 85–90% of babies, and may detect an increased risk of a range of other less common chromosomal conditions.

  • Second trimester serum screening

    This blood test can be performed between 15 and 20 weeks of pregnancy. It can detect approximately 75% of babies with Down Syndrome.

  • Non-invasive prenatal screening test

    Often called “NIPS” or “NIPT”, this is a blood test available from 10 weeks of pregnancy.

    Also known as “the one to find out your baby’s sex”, this screening tool uses a sample of your blood to identify your baby’s DNA circulating in your blood to estimate the chance of your baby having trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). You can also find out the sex of your baby. Some providers also allow you to opt in for screening for additional syndromes or microdeletions at an additional cost. This tool is approximately 99% accurate at identifying Down Syndrome and less accurate with respect to the other trisomies. Your result will indicate a “risk” level, and if a high risk result is obtained, you will be counselled in relation to diagnostic testing options. It is a high-cost screening tool and no medicare rebate is available. Like any screening tool, it has the potential for false positives.

  • Note that most providers prefer for an ultrasound to have been completed either immediately before or as part of this process, so if you have elected not to do a dating scan, an ultrasound may be suggested at this point as part of the process.

What choices do I have for diagnostic genetic testing?

If you have received a high risk result from a screening tool, or have otherwise been recommended a diagnostic test, there are two choices available:

  • Chorionic Villus Sampling (CVS)

    This diagnostic test is available from 11 to 12 weeks of pregnancy. It is an invasive procedure that involves taking a sample of chorionic tissue from your placenta to test for chromosomal conditions or other genetic conditions, including cystic fibrosis. Approximately 1 in every 100 women will suffer a miscarriage as a result of a CVS. Depending on your blood type, you may be recommended anti-D.

  • Amniocentesis

    This diagnostic test is available from around 15 to 18 weeks of pregnancy. It is an invasive procedure involving the extraction of a small amount of amniotic fluid, using a thin needle inserted via your abdomen. It is diagnostic of chromosomal and genetic conditions. Approximately 1 in 200 women will suffer a miscarriage as a result of an amniocentesis. Depending on your blood type, you may be recommended anti-D.

Check out YourChoice — an online decision-aid created by Murdoch Children's Research Institute and James Cook University to support you in deciding what screening tool/s or diagnostic test/s are right for you and your family.

Resources:

*This information does not extend to preconception genetic testing options. To hear a little about updates in that space, listen to this episode of “Knocked Up” about the new medicare rebate available for some preconception genetic tests.