Often referred as the "Down Syndrome Test" - it is not entirely accurate. The right term should be Fetal Aneuploidy Screen and it screens for 3 chromosomal abnormality.
In the first 3 months of pregnancy, there are 2 tests which are offered as screening test for abnormal chromosomes in baby if parents decide to proceed with testing. The two tests are
• First Trimester Screen Test (FTS)
• Non Invasive Prenatal Testing (NIPT)
Both this tests assess risk for :
• Down Syndrome (T21)
• Edward’s Syndrome (T18)
• Patau Syndrome (T13)
What is The First Trimester Screen (FTS)?
This test is a combination of 2 tests - a blood test and an ultrasound. The blood test can be done from 9 weeks to 13 weeks and 6 days ( 9+0 – 13+0 of pregnancy (ideally between 12-13 weeks) and an ultrasound between 11+0 – 13+6 (ideally 12+0- 13+0).
The blood tests investigates for PAPP-A and BHCG levels and the ultrasound assesses the neck thickness of the baby (but can also occasionally pick up other structural issues if present). The two blood levels are calculated in combination with the neck thickness to generate a risk assessment. The results are usually express in a ratio. A high risk result is considered if the risk returns as more than 1:300. This means, in a room of 300 babies there would be 1 baby who has the condition. There are limitations to this test, where up to 5% would have a false positive test (meaning results returns as high risk, but baby does not have Downs Syndrome).
What is a Non Invasive Prenatal Testing (NIPT)
This is a blood test that assesses the baby’s DNA in mom’s blood. The test can be done from 9 weeks of pregnancy. It has a higher accuracy for T21 than first trimester screen, but again is a screening test. Some labs would also offer additional chromosomal screen test with additional cost. The results would return as low or high probability. You can also find out the gender of your baby with this test. The only downside to this test is that there is no ultrasound done usually, which may be useful to assess for any fetal structural issues.
I would usually recommend an early morphology ultrasound (between 13-16 weeks) if you elect to have NIPT.
What if the results return as high risk?
Depending on which test and result, I may discuss and recommend a diagnostic test. It depends on you (and your partner/family’s) preference and choice. A diagnostic test is associated with a small risk of miscarriage. Some couples may elect to proceed with an NIPT test (if they had the FTS only at the start) as it is a safe test. It is best to have this conversation with your obstetrician.