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Antenatal chromosomal testing: screening tests vs. diagnostic tests

With decreasing fertility rates and increasing maternal ages, concern about chromosomal abnormalities in babies has led to improved antenatal screening and detection methods. All woman are now offered screening tests for chromosomal abnormalities in pregnancy, and those who obtain high-risk results are offered diagnostic tests.

Screening tests look at a combination of fetal and maternal findings to give parents the risk of a chromosomal abnormality in their baby. This risk is expressed as a number, e.g. 1/100, which is a 1% chance. The most common screening test is combined first trimester screening (CFTS) – a combination of maternal blood test results and measurement of the nuchal translucency on ultrasound give an estimate of the risk of Down syndrome (trisomy 21), along with two other abnormalities, trisomy 13 and 18. The maternal blood tests are performed at 10-13 weeks of gestation, and the ultrasound at 11-13 weeks.

Newer screening tests include non-invasive prenatal testing (NIPT) – this test detects fetal DNA in the mother’s blood test, and can test for trisomy 13, 18 and 21, as well as abnormalities of the X chromosome. It is usually offered to high-risk patients (e.g. advanced maternal age, abnormal ultrasound findings, high-risk CFTS result), and a cost is involved in performing the test. It is better at detecting these chromosome abnormalities than the CFTS (see table), but as the CFTS includes an ultrasound scan, and therefore can give additional information, CFTS is still the first-line screening test for all pregnant women. Older tests, such as the “triple” or “quadruple” screens (involving maternal blood tests in the second trimester) are less accurate than CFTS or NIPT.

Diagnostic tests are offered to couples who obtain high-risk results in the CFTS and/or NIPT. These tests – chorionic villus sampling (CVS) and amniocentesis – involve collection of placental tissue (CVS) or amniotic fluid (amniocentesis) to assess fetal cells. They are both invasive tests which carry a small risk of pregnancy loss.

Before undergoing any screening or diagnostic test in pregnancy, it is important to discuss the tests, their risks and benefits, and possible outcomes, with your doctor. Not all tests are suitable for all pregnancies, and testing an unborn baby for potential health concerns can be an anxiety-provoking process. It is important to be fully-informed about these tests, and to ask any questions about them, before undergoing them.

Test Screening or diagnostic? When is it done? Accuracy of screening? Risks?



Bloods: 10-13 weeks

Scan: 11-13 weeks

Detection of T21 (Down syndrome) 85-90%; false positive result 5%

No risks to mother or  baby



From 10 weeks

Detection of T21 >98%; false positive result <1%

No risks to mother or baby



10-14 weeks

Pregnancy loss up to 1% for transabdominal CVS, up to 2% for transcervical CVS

Placental mosaicism (“mixed cells”) 1% – result then difficult to interpret, recommend amniocentesis

No result – rare, repeat CVS or do amniocentesis



After 15 weeks

Pregnancy loss up to 1%

Infection <1/1000

No result – rare, repeat procedure

Mater Mother’s Hospital patient information brochures:References:

  • Pregnancy – testing to Down syndrome and other chromosomal abnormalities
  • Pregnancy – chorionic villus sampling (CVS) and amniocentesis

Non-invasive prenatal testing (NIPT) – Dr Glenn Gardener, Director, Mater Centre for Maternal Fetal Medicine

To speak directly with a team member please call 07 3188 5000