First Trimester Screening For Down Syndrome

First Trimester Screening & Non-Invasive Prenatal Testing - What it is

Down syndrome is the most common cause of learning difficulties in humans. It occurs in approximately 1 out of every 700 babies.

All individuals with Down syndrome will have some degree of learning disability. They are also at higher risk of medical problems such as heart defects, abnormalities of the digestive system, problems with hearing and vision,

seizures, thyroid problems and childhood blood cancer. Life expectancy is around 40 to 60 years.

The severity of the condition varies from person to person. Antenatal testing can tell if a baby has Down syndrome, but cannot predict how severely an individual will be affected.

First Trimester Screening & Non-Invasive Prenatal Testing - Causes and Risk Factors

Down syndrome arises due to the presence of extra genetic material (known as a “chromosome”) in a person’s cells. This additional genetic material affects the way the brain and body develop.

Women at higher risk of having a baby with Down syndrome include those:

Who are older when they get pregnant
Have a previous child with Down syndrome
Have siblings with Down syndrome

However, anyone can have a baby with Down syndrome, including women who are younger or who have no history of Down syndrome in the family.

Figure 1. Relationship of risk of Down syndrome with respect to maternal age.

Figure 1. Relationship of risk of Down syndrome with respect to maternal age.

First Trimester Screening & Non-Invasive Prenatal Testing - Diagnosis

SHOULD I TEST FOR DOWN SYNDROME?

Deciding to test for Down syndrome is a personal choice. The advantages of diagnosing Down syndrome before birth include preparing the couple for the birth of a child with special needs and allowing them to decide whether to continue with an affected pregnancy.

HOW CAN I TEST FOR DOWN SYNDROME?

There are two types of tests - screening and diagnostic.

Diagnostic tests confirm whether the baby has Down syndrome. They are however invasive and carry a risk of miscarriage of approximately 1%. Please refer to “Chorionic Villus Sampling” and “Amniocentesis” for more details.
Screening tests estimate the chance of your pregnancy being affected by Down syndrome. This will help you to better decide if you want to proceed with an invasive diagnostic test.

Advantages of screening tests

Provide a woman with more information by giving her a personalised estimate of the risk of Down syndrome in her baby
No risk of miscarriage

Disadvantages

May cause unnecessary anxiety due to false-positive results, which happen when babies are identified as high-risk when they are actually unaffected.
May lead a woman to opt against diagnostic testing that would have revealed problem due to false-negative results. This occurs when babies are identified as low risk when they are actually have Down Syndrome.

WHAT ARE MY OPTIONS FOR SCREENING?

Several screening methods are available for Down syndrome.

These include

First trimester screening: Please refer to the next section.
Non-invasive pre-natal testing (NIPT): This can be performed from 10 weeks of pregnancy. A blood test is used to detect fetal cell free DNA within the mother’s blood. Approximately 99% of Down syndrome cases can be detected via this method.
Second trimester maternal serum screening: This is done between 15-20 weeks of pregnancy. It involves a blood test to measure the level of several proteins in your blood. The results are combined with your age to work out your individual chance of having a baby with Down syndrome. Around 65% of Down syndrome cases can be detected via this method.
The 18-20 week scan is a detailed ultrasound examination to look for structural abnormalities that may suggest the possibility of Down syndrome in the baby. However, up to 50% of babies with Down syndrome have a normal ultrasound scan. This means that the scan on its own may only detect half of the babies with Down syndrome.

First Trimester Screening & Non-Invasive Prenatal Testing - Treatments

WHAT IS FIRST TRIMESTER SCREENING (FTS)?

First trimester screening is performed between 11-14 weeks of pregnancy when the baby measures between 45 - 84mm in length. There are two parts to the test and a computer software will combine the results with the mother’s age to give an estimate of the risk for Down syndrome.

The two components of FTS are -

An ultrasound scan to measure the nuchal translucency (NT), which is the amount of fluid at the back of baby’s neck. NT measurement can detect 80% of babies with Down syndrome if performed alone. The ultrasound scan will additionally allow your doctor to screen for physical abnormalities in the baby.
A blood test to measure the level of two proteins - pregnancy associated plasma protein A (PAPP-A) and human chorionic gonadotropin (hCG) - in the mother’s blood. This improves the detection rate for Down syndrome to 90% when performed in combination with the ultrasound scan.

Figure 2. Nuchal translucency (seen between the two white + markings) of the fetus.

Figure 3. Thickened nuchal translucency (measurements within the 2 white + markings) in a baby with Down Syndrome

You may opt to do (1) alone or both (1) and (2) together, which is known as the combined test.

Table 1: Summary of currently available screening tests for Down Syndrome

HOW DO I PREPARE FOR FTS AND WHAT CAN I EXPECT?

It is important to keep to your scheduled appointment, as changing the appointment date may result in the baby being too small or too large for the FTS to be accurate. There is no need for any special preparation for FTS. You can eat and drink on the day of the test.

The ultrasound scan will usually be performed through the abdomen. In some cases, accurate measurement of the NT cannot be achieved because of the position or size of the baby. If the baby is <45mm in length, you will be given another appointment for repeat scanning. If the baby is >84mm in length, alternative methods of screening will be discussed with you.

ARE THERE ANY RISKS IN PERFORMING FTS?

No, FTS will not cause any risk to your pregnancy.

HOW DO I INTERPRET MY FTS RESULTS?

Your results will be reported in the form of an estimated risk.

For example –

Mdm A undergoes FTS. Her estimated risk of having a baby with Down syndrome is 1 in 10. This means that among 10 similar pregnancies with the same ultrasound and blood protein measurements, 1 pregnancy will eventually result in the birth of a baby with Down syndrome, while the other 9 pregnancies will result in the birth of babies unaffected by Down syndrome.

WHAT IS A HIGH-RISK RESULT AND WHAT HAPPENS IF MY RESULTS ARE HIGH-RISK?

A “high-risk” or positive result is when the chance of the baby having Down syndrome is higher than 1 in 300 on FTS. Please remember that FTS only tells you the chance of the baby having Down syndrome, and does not confirm

if the baby has Down syndrome or not. Around 5% of women doing FTS have a false-positive result, meaning that

the test result is positive but the baby does not actually have Down syndrome.

Your doctor will discuss your results with you and explain your options. These may include doing another screening

test such as NIPT, or performing a diagnostic test such as chorionic villus sampling or amniocentesis. Diagnostic tests are the only way to confirm the diagnosis but are associated with a small risk of miscarriage of about 1%.

WHAT IS A LOW-RISK RESULT AND WHAT HAPPENS IF MY RESULTS ARE LOW-RISK?

Most women who undergo FTS have a low risk result. This does not mean that the baby definitely does not have Down syndrome, but suggests that the likelihood of the baby being affected is small.