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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.