How Is Down Syndrome Suspected During Pregnancy?

It is only natural for expectant mothers to worry about their baby’s health and growth.There are a number of tests that can be done in pregnancy for this. One that you would commonly hear about is the screening for Downs syndrome.Therefore it is useful to know and understand what it is and why it needs to be done.

What Is Downs Syndrome?

Downs syndrome or Trisomy 21 is a genetic condition in which children are born with an extra chromosome.

It is estimated that about one in 1,000 baby is born with Down’s syndrome. Women of any age can have a baby with Down's syndrome, although the chance is higher in older women.

This condition results in learning difficulties which can be mild to severe along with some other problems like low muscle tone, heart defects etc. Understandably then, bringing up a child with Downs syndrome can have a major impact on the family.Having said so, many children with downs syndrome are able to lead long and productive lives.

Knowing that your baby will be born with this condition helps families to make decisions about continuing or terminating the pregnancy in time.

What Tests Are Done During Pregnancy to Check For Downs Syndrome?

There are screening tests to assess risk and diagnostic tests to confirm a diagnosis.

Screening Tests

These are usually offered to all women and are done in the first 14 -20 weeks of pregnancy. These tests do not give a definite answer. They simply tell you whether your baby may have a higher chance of having Downs Syndrome. The screening tests include:

• ultrasound scans
• blood tests

A "screen negative" means that you are extremely unlikely (but not impossible) to be carrying a baby with Downs Syndrome. 

First Trimester Screening

From 11 to 13 weeks of pregnancy, a special ultrasound scan called a nuchal translucency (NT) scan can be performed. This measures the fluid under the skin at the back of the baby's neck to determine the risk of Down's syndrome. There is also a blood test called the Double marker test which measures levels of markers like hCG and PAPP-A that vary in babies with Down syndrome.

Second Trimester Screening

This is based on blood tests which are carried out between 15 and 21 weeks of pregnancy. The tests measure the level of several "markers" in the blood like

• b-hCG
• AFP
• unconjugated estriol and
• Inhibin A

Depending on the number of markers tested, there are variations of this test that are available such as Double/Triple or Quadruple screening test. These tests aren't as good at detecting babies with Down's syndrome as the first trimester tests. [Read More - Genetic Screening Test in Pregnancy]

Combined First & Second Trimester Screening

An Integrated Test combines ultrasound scan results and blood tests for markers in both trimesters to make it a more effective assessment.

In addition to the combined test, there is also the non-invasive prenatal testing (NIPT) which is the most accurate non-invasive screening test. NIPT involves a single blood test, and can be done from 10 weeks. Mother’s blood is analyzed for fragments of the baby’s DNA to find out the chance of having a baby with Down’s syndrome. It has detection rates as high as 99 per cent and is an expensive test.

What If The Screening Tests Are Positive?

Diagnostic tests are then offered to women if their screening tests are positive. These are invasive procedures – CVS ( chorionic villu sampling) done in early pregnancy and amniocentesis. They give a definite answer but also carry a risk of causing miscarriage.

The tests, though not compulsory, are done to check whether your baby has Downs syndrome or not. If it is confirmed, you can then with the help of your doctor and genetic counselor make an informed decision that you and the family are comfortable with.

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