Down syndrome occurs if a baby has three copies of its 21st chromosome instead of two copies. A chromosome is a bundle of genetic material, and if you have too many or too few of these bundles, severe abnormalities can result. In the case of Down syndrome, these include abnormalities of the intellect, heart, thyroid and other endocrine systems, facial appearance and others.
The likelihood of you having a baby with Down syndrome increases with age, as shown in the graph below.
Through consideration of your age and additional tests, it’s possible to detect early in the pregnancy whether your baby might be at higher risk of having Down syndrome.
The additional tests are Combined First Trimester Screening (CFTS) and Non-Invasive Prenatal Screening (NIPS).
It is important to realise that both of these tests are considered to be ‘screening tests’. This means they won’t tell you ‘yes, your baby has Down syndrome’ or ‘no, it definitely does not have Down syndrome’. They will, however, say if your baby is at high risk of having Down syndrome, in which case you should have a diagnostic test.
A diagnostic test is one that will tell you ‘yes’ or ‘no’.
NIPS is the best test available to screen for Down syndrome. It directly tests the baby’s cells that can be found in the mother’s blood stream. As such, all that is required is a maternal blood sample. The results are almost as accurate as CVS (chorionic villi sampling) or amniocentesis, both of which are invasive, requiring the insertion of a needle into the pregnancy. NIPS is quite expensive, costing about $480, and no Medicare or insurance rebate applies. NIPS will also report whether the baby is a boy or a girl, although we will only tell you this result if you want to know.
CFTS requires a blood test at 10 weeks and an ultrasound between 12 and 13 weeks. The blood test looks at hormone levels that are produced by the placenta, while the ultrasound assesses the thickness of a fluid pocket behind the baby’s neck. Thus, CFTS looks at indirect markers of Down syndrome, whereas NIPS looks directly for the chromosomal abnormality. The cost of the blood test component of CFTS is $110, and no Medicare rebate applies.
The 12 week anatomy ultrasound is absolutely essential when doing CFTS. While it is not a requirement for NIPS, it is still an essential test for other reasons.
What test should I do?
The vast majority of patients in my practice prefer the NIPS; however, it is less important if you are younger than 30 years old. I will discuss these options with you in detail at your first pregnancy appointment.
It’s important to realise that there are lots of brands of NIPS — my recommendation is to opt for percept NIPS through VCGS, Victoria’s premier genetic testing laboratory.