Screening for Down Syndrome during Pregnancy

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What is Down syndrome?

The majority of babies are born healthy, but around 1% (1 in a100) are born with severe physical or mental handicap. Downs syndrome is one of the common conditions where the baby is born with an extra chromosome (47 instead of 46). The extra chromosome is number 21 thus the condition is known as Trisomy 21. It is more common in older mothers, but it can occur at any age.

How can I find out whether my baby has Down syndrome?

There are two ways of finding out during pregnancy whether the baby has Down syndrome. There are invasive tests which are either chorionic villus sampling (CVS) or amniocentesis. There are as well screening tests which involve taking blood from the mother and ultrasound examination of the fetus known as nuchal translucency (NT).

What exactly are invasive tests?

Invasive testing for Down syndrome involves introducing a needle through the abdomen into the womb under ultrasound guidance. CVS which means taking a small sample from the placenta “after birth”, can be done from 11 weeks onwards. The procedure is done under local anaesthetic. The other test is amniocentesis where a sample of the fluid around baby is obtained using a fine needle. The fluid contains the baby’s’ cells which have been shed from the skin. Amniocentesis can be done after 15 weeks of pregnancy, to minimize the risks associated with the procedure.

How accurate are invasive tests

Both CVS and amniocentesis are very accurate tests, as the samples obtained are sent to specific labs to count the number of chromosomes. The cells obtained are cultured, a process which takes about 10 days. The chromosomes are counted to find out whether the baby has 46 chromosomes or not. It is possible if the parents wish to find out the sex of the baby as well. There is a quicker test called FISH, this test can give results within 3 days.

Are there any risks with amniocentesis or CVS?

With any invasive testing there is a risk of miscarriage, the chances of it happening are 1in 100 cases. Miscarriages can occur within 2-3 wks after the procedure, which may be due to infection or a faulty technique. There are theoretical risks of limb deformities and early labour as well, but they are not common and usually occur with early amniocentesis before 15 weeks.

What is the nuchal translucency scan?

This is an ultrasound examination which is done between 11 and 14 weeks of pregnancy. The fluid at the back of the baby’s neck is measured. All babies have a bit of fluid at the back of their neck, but in babies with Downs syndrome the fluid is increased. The measurement obtained is used to work out the chances of the baby having Down syndrome. The scan can as well confirm the dates, diagnose twins, check that the baby is developing normally and study the fetal anatomy at this stage.

The nuchal translucency scan can pick up about 80% of Downs syndrome babies. The results are presented to the mother as a risk value e.g. 1 in 700. The test takes into consideration the mother’s age when calculating the risk. The test will not tell the mother whether her baby has Downs syndrome or not, but will help the mother to decide whether to have invasive testing or not. Invasive testing will be offered to mothers if the result is 1 in 250 or worse.

What about blood tests to diagnose Down syndrome?

This test is widely available in Egypt where blood is taken from the mother at 16 weeks of pregnancy to find out the chances of baby having Down syndrome. The blood which test for certain pregnancy hormones, should be sent to specific laboratories which can perform the test and provide the mother with a risk value. The blood test can pick up about 65-70% of babies with Down syndrome. However, false positive results can occur in 5% of women. The results are used as a guide to the mother to decide whether to undergo any further invasive testing.

First trimester blood testing

Blood test taken from the mother can be used to measure two hormones (free ?-hCG and PAPP-A). In the presence of Downs syndrome the free ?-hCG tends to increase and the PAPP-A is decreased.

Combining the results of screening tests

The NT results and blood results performed at 16 weeks cannot be combined to give a final result. But the first trimester blood test can be used in conjunction with the NT measurement to provide the mother with a risk. The results are more accurate when combining the two tests together as the pick up for Down syndrome is 90%.

What about the fetal nasal bone?

The presence of the nasal bone when examining fetal profile at 11-14 weeks is reassuring. The pick up for Downs syndrome is up to 95% if the nasal bone is used in conjunction with the nuchal translucency and first trimester biochemistry.

Many pregnant women are becoming aware of the screening tests available nowadays in Egypt. It is the job of your Obstetrician to provide you with all the necessary information to help you make your own decision whether to undertake any tests or not. Your doctor is not in a position to tell whether to proceed with any of the tests mentioned above, he can only guide and advise but the final decision has to be yours together with your husband.

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