Combined Screening Test
The combined screening test is also known as the first trimester screening test: it is done during the first trimester (between 11 and 14 weeks), and combines information from the ultrasound examination of the fetus and the blood of the mother.
In many parts of the world, the combined screening test is the current standard for Down syndrome screening. The test can be done in a particular window of pregnancy: when the length of the fetus is between 45mm and 84 mm only. Before and after this window, the test is inaccurate and hence not offered. Read the FAQ about Down Syndrome screening provided below to know more about the combined screening test
Quadruple Screening Test
The quadruple screening test (QST) is preferred over the triple screening test (TST), when it comes to choosing between the two. Essentially, the test consists of testing mother’s blood for hormones produced by the fetus and placenta. In triple test, three hormones were tested – human chorionic gonadotropin (hCG), alpha fetoprotein (AFP) and unconjugated estriol (UE3). Subsequently, a fourth hormone was added to the test – dimeric inhibin A (DIA) and the test became known as quadruple screening test. The advantage of QST over TST, is the higher detection rate while the false alarm rate is lesser.
The QST is performed when the fetal head measurement called the BPD (Biparietal Diameter) is between 32 and 52mm, since during this time the hormones tested have a predictable levels. Therefore the hormones level in a particular patient can be easily compared to the expected level for that stage of pregnancy.
At The Fetal Clinic, we offer QST as a screening test for Down syndrome if you have not undergone the combined screening test.
Non Invasive Prenatal Test
The non-invasive prenatal test (NIPT) is also known as non-invasive screening (NIS), DNA-test, cell free fetal DNA test (cf-fDNA). There are a handful of commercial laboratories world-wide that have the first hand technology to provide the NIPT.
The Genetic Sonogram is a detailed fetal examination through ultrasound looking for 8 subtle clues. In about 50% of Down syndrome fetuses, there may be two or more of these features which will then alert us to offer the definitive test – amniocentesis.
Usually, the genetic sonogram is not offered as a standalone screening test for Down Syndrome because of the low detection rate and high false alarm rate. It is combined with the QST, especially when the results of the QST comes in the borderline zone.
What is Down syndrome?
Normally humans have 46 chromosomes in their cells. Instead, if there 47 chromosomes (due a extra copy of 21st chromosome), Down syndrome occurs. These children may have variable degree of intellectual handicap, problems of heart, intestine, and other health issues. This particular type of intellectual handicap was described by Dr John Langdon Down in 1866 and came to known after him
How do we confirm if a fetus has Down syndrome?
The only certain way to know if the fetus has Down syndrome or not is to study the fetus’s chromosome. For this, a direct test of the baby’s fluid (amniocentesis) or placenta (chorionic villus sampling) needs to be done. However, 1 in about 200 to 500 such tests may result in a miscarriage and hence it is not advised for everybody. Only for those fetus that have a high chance of having Down syndrome, this test is recommended
What does the first trimester screening (FTS) attempt to predict?
Although there are hundreds of genetic problems, most of these are rare. The condition called “Down Syndrome” occurs in about 1 in 600 births. The FTS test attempts to predict this condition.
What is the use of the FTS?
With the FTS test, we cannot determine if the baby has or has not Down syndrome (it is not a diagnostic test). However, we can determine what chance does your baby has to have Down syndrome (screening test or indirect test). For those fetus that have a high chance, CVS or amnio would be recommended.
What are the parts of the FTS?
The first component is a detailed ultrasound examination of the fetus looking for a) Length of the fetus (Crown Rump Length) (b) Fluid behind the baby’s neck (c) Presence or absence of baby’s nose bone and (d) baby’s heart beat rate. Apart from these information, the scan also looks in detail about the baby’s activity and organ formation according to the age of the fetus. The second component is a blood test (usually a finger-tip prick; sometimes blood drawn from a vein) to measure certain hormones produced by the placenta. Using information from both components, the computer programme calculates the chance of being affected with Down syndrome based on the mother’s age and weight.
How do we interpret the FTS report?
The result is usually given as a chance calculation (also known as risk calculation in medical terms). For example, if the test result says Final Risk 1: 970, this means that out of 970 pregnancies that have the same scan measurements and blood test measurements, there is going to be 1 fetus with Down syndrome (i.e this fetus has a 1 in 970 chance of being affected). In medical practice, we would recommend going ahead with direct testing when the chance is greater than 1 in 250 (for example, when the final risk is 1 in 150 or 1 in 75 etc…)
How long does this ultrasound examination take to complete?
Unlike other scans, the fetus has to be in a very specific position to accurately take measurements. So, it may be required to wait for the fetus to attain this position. In about 2% of fetuses, it may not be possible to take satisfactory measurements even after repeated attempts due to several reasons. In that case, only the blood test will be taken for the chance calculation.
What is the correct time to undertake the FTS?
The appropriate time to undertake FTS is when the baby’s length (CRL) is between 45 and 84 mm; this usually corresponds to 11 weeks and 14 weeks from the last menstrual period
Can we detect all fetuses with Down Syndrome with FTS?
As a result of 10 – 15 years of research in fetal medicine, this strategy (of all women undergoing FTS, and in those with final chance of 1 in 250 or greater performing amnio / CVS) is shown to detect about 85 – 90% of all Down syndrome fetuses.
Also, you should be aware that not all fetuses that get a score of greater than 1 in 250 will have Down syndrome. In fact, among fetuses that have a risk (chance ) of greater than 1 in 250, only 2 – 3% will have Down syndrome
Where can I find more information?
To know more about invasive tests (CVS / Amnio), please visit the page on invasive testing. To listen to an explanatory video about screening for Down Syndrome visit www.fetalmedicine.org website.
What to do if I miss my first trimester screening test?
The First Trimester Screening Test (FTS) is accurate when performed between fetal length of 45 and 84 mm, that roughly corresponds to 11 weeks and 14 weeks. If for any reason you had missed the window to undergo the test and would still like to be screened for Down Syndrome, the next best option is to go for an ‘early pregnancy anomaly scan’ followed by the Quadruple Screening Test (QST). Although not as accurate as the FTS, the QST along with a detailed scan is sufficient to reassure most pregnancies.
Can I undergo the NIPT as a first choice test?
The NIPT is advised as a second step test in very select circumstances rather than as the first line due to two reasons: a prohibitively high cost and the need for a high quality scan to exclude major fetal defects. At The Fetal Clinic, we offer NIPT in select situations only after a detailed counseling regarding the pros and cons of the NIPT compared to other options that would be applicable in that particular individual.