Screening tests in general are able to utilise a range of information about you and your baby to give a more accurate estimate of what risk there is of your baby having the condition being screened for. The screening test carries no risk to your pregnancy, but is unable to give an exact answer about whether your baby is actually affected or not. The purpose therefore of the screening test is to enable you to make a better informed decision about whether or not to have a diagnostic test performed.
If the screening test is low risk, in general I would counsel you that diagnostic testing is not necessary. If the screening test says your baby is at increased risk of a chromosomal problem (for example Downs syndrome) we would then have a discussion about what that risk is and what your options are for further investigating that. Whether or not you decide to proceed with any subsequent testing is entirely up to you however there are some risks to pregnancy for diagnostic tests and this is why they are not performed routinely.
1. The Combined First Trimester Screen (CFTS)
A combined first trimester screen (CFTS) is a combined blood test and ultrasound scan which is used as a screening test for abnormal chromosomes- the most well known one is Downs syndrome. The nuchal translucency test can be done between 11 weeks 3 days and 13 weeks and 6 days of pregnancy (ideally at 12 weeks). The blood test is most informative if collected during the tenth week of pregnancy, however it can be collected between 9 weeks and 12 weeks 6 days of pregnancy.Two hormones in your blood are measured. The result is sent to the Radiology service where you are having your scan. The blood test increases the accuracy of the test to about 90% meaning for 100 babies affected 90 will be detected by this test. The test has a false positive rate of 5% meaning for every 100 scans done the test will be abnormal 5% of the time when in fact the baby is normal. If the test suggests your baby is high risk a diagnostic test is offered which is able to see if your baby really is affected or not. This test is optional and not all women wish to know this information and that is fine, however the other advantage of a scan around 12 weeks is it gives an opportunity to pick up major abnormalities with the baby which would otherwise not be identified until the next scan at 18-20 weeks.
2. NIPT – Non Invasive Prenatal Testing
This is a new screening test which has recently become available in Australia. This test is performed by taking a blood sample from mother and collecting genetic information about the baby from the mother’s blood stream. It is designed to identify babies with the most common abnormal chromosome variants and can also tell you your baby’s sex. The advantages of the test are that it poses no risk to your baby, can be done from10 weeks of pregnancy with a result back in about 1 week and it can be used for twin pregnancies, It gives a result 99% of the time and has a better detection rate than the CFTS for Downs at around 99%. This test has been years in development and is now available for use in pregnancy however it is expensive and is not covered by Medicare. Currently the cost is around $600 because the specimen is processed in the Unites States. Even if this test result is abnormal, it is still recommended a diagnostic test be performed to confirm the abnormality detected prior to any action being taken.
Diagnostic tests aim to diagnose the condition of the baby. That may be confirmation that your baby has normal chromosomes or confirmation that your baby has a chromosomal abnormality. Two tests are available and which one is recommended depends on how far along the pregnancy is when the test is performed and what kind of testing is required. Usually the test is undertaken by a the Feto Maternal Specialist and usually they would advise you about what test to have. Both involve a very small needle being inserted through your abdominal wall and the uterine wall into the sac the baby is in. One collects cells form the placenta (Chorionic Villus Sampling – CVS) the other fluid from around the baby (Amniocentesis). The test sounds a bit scary but it really isn’t any worse than a blood test, but the problem with these tests is that there is a 1% risk of miscarriage related to the procedure. That risk is independent from whether or not your baby is normal so rarely a normal baby will miscarry as a result of amniocentesis or CVS.
Both tests are performed under ultrasound guidance to reduce the risk to the baby and CVS is usually done earlier from 11-13 weeks whereas amniocentesis is usually done between 15 and 20 weeks. The sample taken is sent to a laboratory and the cells are tested. A quick result can be available within 48 hours called FISH. (Flourescent In-Situ Hybridisation) FISH gives a quick result for the common chromosome problems and growing the cells and getting a formal culture result takes 2 weeks.