Pregnancy is a wonderful time, filled with excitement and anticipation of a new addition to the family. The early weeks of pregnancy can be exciting, but anxious, even so for first-time mothers. Like most expectant mothers,   you may want reassurance that your baby is normal and healthy. We as doctors are as eager as your family and you to ensure that you have a smooth pregnancy and a healthy baby.

The majority of pregnancies end happily with the birth of a healthy child. At the same time, however, every woman has a small inherent risk of having a baby with a chromosomal abnormality. Prenatal screening is designed to identify women who are at higher risk of delivering a baby with such abnormalities. Scientific research over the last 2 decades has shown that an ultrasound scan between 11-14 weeks can help detect up to 85-90% of such chromosomal abnormalities.

What are chromosomal abnormalities?

Inside the cells of our bodies, there are tiny structures called chromosomes. These chromosomes carry the genes that determine how we develop. Most people have 23 pairs of chromosomes in each of their cells. When our bodies produce the cells needed to make babies, the chromosome pairs divide and rearrange themselves. Sometimes these pairs of chromosomes accidentally do not divide correctly, and this can cause the baby’s cells to have an extra copy of chromosome 21. This causes Down’s Syndrome (also called Trisomy 21). The extra chromosome cannot be removed from the cells, so there is no cure for the condition.

What is Down’s Syndrome?

Down’s syndrome is the most common chromosomal abnormality, where the child is born with an extra chromosome 21. Down’s syndrome occurs in 1:700 pregnancies all over the world. The word ‘syndrome’ means a collection of characteristic features. People with Down’s Syndrome have learning difficulties but some are more severely affected than others. There are also physical problems associated with the condition; for example, 40% will have a heart abnormality. While mothers over the age of 35 are at increased risk of having babies with Down’s syndrome, more than 75% of babies are born to mothers who are less than 35 years old.

Will the tests tell me for certain if my baby has Down Syndrome?

We begin by offering all women a test that carries no risk of miscarriage. This type of test is called a screening test. Screening tests do not give a definite answer, but they do help us to tell which babies have an increased risk of having Down’s Syndrome. We then offer diagnostic tests to women with an increased risk. These are tests that give definite information but they do carry a small risk of miscarriage. This is why we only offer a diagnostic test to women who have been found to have an increased risk.

Combined first-trimester screening test:

At this clinic you can choose to have a screening test for Down’s Syndrome that involves a scan and blood test. This test can be taken from 11 to 14 weeks of pregnancy.

All babies have a very thin film of fluid under the skin at the back of their neck at this stage of pregnancy. We can measure this as part of the scan and this is known as the nuchal translucency measurement.

A blood sample is taken from the mother’s arm during the same appointment as the scan. The blood test measures the amount of some substances that have passed to the mother from the baby. If a baby has Down’s Syndrome, different amounts of these substances can be found in the mother’s blood. The result of the blood test, together with the mother’s age, weight, and stage of pregnancy is combined with the nuchal translucency measurement, to work out the chance of your baby having Down’s Syndrome. This is the combined test and the result is given as a risk figure. Risk can also be calculated for twin pregnancies.

Second-trimester screening:

If you wish to have Down’s Syndrome screening, and your pregnancy is 14 weeks and over when you come to the first-trimester screening clinic, you will be offered second-trimester screening. This is a blood test and can be carried out between 14 – 20 weeks of pregnancy. The result is given as a risk figure.

What is a high-risk (screen-positive) result?

Both the screening tests for Down’s Syndrome give results in the form of risk; for example ‘one in 100’. These numbers tell us how likely it is that the baby has Down’s Syndrome. For example, the result ‘one in 100’ means that there is one chance in 100 that the baby has Down’s Syndrome. If your screening result is above 1:250, we will offer you further tests. This is known as a high-risk (screen-positive) result.

What is a low-risk (screen negative) result?

If the screening test shows the risk of your baby having Down’s Syndrome is lower than 1:250, this is known as having a low-risk (screen negative) result and we will not offer you further tests. Most screening test results fall into this category. It is important to understand that a low-risk (screen negative) result means that it is unlikely that your baby has Down’s Syndrome. Some babies with Down’s Syndrome are not detected by screening tests. Overall, about 1 in 4 babies with Down’s Syndrome are not detected by screening tests. It is very important to remember that screening tests fit into a two-stage process, so if you get a high-risk (screen-positive) result, it means we will offer you more information and tests. It does not mean that your baby definitely has Down’s Syndrome.

What happens if I have a high-risk (screen-positive) result?

If the result of the screening test shows the chance of your baby having Down’s Syndrome is greater than 1:250, we will offer you a diagnostic test. Overall about one in 33 women screened have a high-risk (screen positive) result and are offered a diagnostic test.

Diagnostic tests for Down’s Syndrome

There are two diagnostic tests offered for Down's Syndrome,

• Chorionic Villus Sampling (CVS)
• Amniocentesis (amnio)

Are these procedures safe?

These procedures do carry some risk, and this is why we don’t offer them to everybody. For every 1000 women who have amniocentesis, one will miscarry. And for every 1000 women who have CVS, one or two will miscarry.

• Amniocentesis
• CVS (Chorionic Villus Sampling)