There are a number of diagnostic and screening tests available to the expectant mother. Some are routinely used and others are specialised and not covered by Medicare.
None are compulsory. Before undergoing any testing, it is a good idea to think about the implications of a result that indicates a likelihood of abnormality.
No test, or combination of tests, will detect all birth defects or medical conditions.
Screening tests during pregnancy are a matter of choice. Until recently, there was only a single test available in the first and second trimesters. Now there is an option for a more accurate test previously unavailable.
First trimester combined screening test – 90% accuracy:
- Blood test at 10-12 weeks of pregnancy
- Ultrasound at 11-13 weeks of pregnancy to measure the baby’s the nuchal translucency
- Calculation of risk taking into account the maternal age
The results of this test are not conclusive but instead reveal the likelihood of the baby being born with chromosomal changes including Down syndrome (trisomy 21) and Edwards syndrome (trisomy 18).
If the risk is higher than normal, genetic counselling is offered to decide whether to undergo more conclusive diagnostic testing.
Since the test is optional, there is an out-of-pocket cost associated with the blood test and the ultrasound.
Non-invasive prenatal test (NIPT) – 99% accuracy:
This involves analysing fragments of DNA from the baby, which are naturally present in the mother’s blood stream. A blood sample is taken at 10 weeks and sent to the United States for analysis. There are several NIPTs currently available and the names differ, depending on which company has developed them. Tests zcurrently available in Australia include Harmony, Panorama, MaterniT21Plus, Verifi, Nifti and Percept.
NIPTs are optional: they are not covered by Medicare or private health insurance, and require a referral from your GP or obstetrician. The cost is about $500 to $1000 and results take 2-3 weeks. Percept is available in locally in Victoria through Victorian Clinical Genetics Services, costs $490 and provides results in 3-5 days.
Reliability for Down syndrome testing is 99%. Thefollowing chromosomal abnormalities are also tested but reliability is highest for Down syndrome:
- Trisomy 13 – Patau’s syndrome
- Trisomy 18 – Edwards’ syndrome
- Trisomy 21 – Down syndrome
- Sex chromosome abnormalities involving X and Y chromosomes.
A 12-week ultrasound is still recommended to monitor baby’s development and other malformations. NIPTs are not suitable for twins or multiple births.
As with other screening tests, the results are reported as a probability and, if a high risk is detected, only a diagnostic test will confirm the abnormality.
All NIPTs can be used as screening tests for low-risk and high-risk women. As the chance of genetic disorders increases with age it’s expected that older women would benefit most from testing and may be able to avoid invasive tests.
Second trimester maternal serum screening test – 85% accuracy:
- Blood test at 14-20 weeks of pregnancy (ideally at 15-17 weeks)
- Calculation of risk taking into account the maternal age.
This is not as accurate as the combined first trimester test but it can still detect increased risks of chromosomal alternations (such as Down syndrome) and additionally can detect neural tube defects (involving the spinal cord or brain such as spina bifida).
Test results – calculate risk not certainty
A low risk result still carries a risk (albeit small) of having a baby with Down syndrome Even a test result that indicates an increased risk does not mean that the baby will be born with Down syndrome – it simply provides an indication that further diagnosis tests are warranted (but not compulsory).
Some pregnancies are at increased risk of problems. For example, there may be a family history of an inherited disorder or the mother may be older or may have already given birth to a baby with chromosomal or other disorders. In other cases, the pregnancy screening tests may have picked up an abnormality.
Diagnostic tests in the second trimester may be invasive, however, they are more accurate than screening and they aim to provide (but cannot guarantee) a firm diagnosis.
Both tests carry a small risk of complications including miscarriage estimated at 1:100 to 1:200. Results take 2-3 weeks.
Chorionic Villus Sampling (CVS) at 11-13 weeks of pregnancy (no later than 13 weeks)
A thin needle is guided under ultrasound through the mother’s abdomen and into the tissue that forms part of the placenta (chorionic villi). This tissue is then examined in a laboratory for chromosomal alterations as well as any genetic conditions known to present a risk due to family history or prior screening.
Amniocentesis at 15 -16 weeks of pregnancy (no later than 20 weeks)
A needle is guided under ultrasound into the amniotic sac and the cells in the amniotic fluid are examined for abnormalities such as Down syndrome, cystic fibrosis and spina bifida. The test reveals the sex of the baby.
The risks are low but include infection, injury to the baby and a 1:200 risk of miscarriage.
Ultrasound can be performed at any time during a pregnancy particularly if there are unusual symptoms such as vaginal bleeding or reduced fetal movements.
Ultrasound (second trimester)
A detailed ultrasound is routinely carried out at 18-20 weeks to check growth, the amount of amniotic fluid, the position of the baby and the size and location of the placenta. Physical problems with the heart, kidney, cleft palate and limbs are capable of being detected as are neural tube defects. This is the ultrasound at which the sex of the baby can usually be determined.
Ultrasound (third trimester)
An ultrasound may be offered to check the baby’s growth, fluid levels around the baby and the positioning of the placenta.
Screening late in pregnancy – Strep B
Group B streptococcal (GBS) bacteria can cause a wide range of illnesses. One to four out of every 1000 newborns contract a GBS infection from their mothers during childbirth. Life-threatening complications include blood poisoning, meningitis and pneumonia.
There is no standard screening procedure for GBS in Australia and hospital protocols vary. Since an earlier negative swab can later become positive, it appears advisable for screening of a pregnant mother to be left until late pregnancy (35-37 weeks). Testing is carried out by vaginal swab.
A pregnant woman with GBS can be easily treated with intravenous antibiotics when labour begins, to reduce the risk of GBS infection in the newborn.
Optional genetic testing – not covered by Medicare
Cystic fibrosis (CF) is an inherited condition, affecting one in every 2500 babies born in Australia.
Healthy adults may be carriers of the gene without being aware of their status. Both parents have to be carriers for there to be a risk of the CF gene being inherited by the child. Ideally, testing is carried out prior to pregnancy and is particularly important in those with a family history of CF. Very accurate carrier risk assessment is possible if the family mutations are known but otherwise it is considered to be about 90% accurate.
Testing is via a mouth swap and can be requested from a GP or obstetrician.
As with all screening, consideration should be given to the need for genetic counselling before undergoing testing and the options available if a foetus is diagnosed with CF.
*The material provided in our information sheets is for general knowledge only and is not a substitute for independent legal advice. For further information about the issues affecting you, please contact one of our experienced and professional lawyers for expert advice.