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Diagnosing Down’s syndrome before birth

Wednesday, April 26th 2017. | Down's Syndrome

Down’s syndrome is a genetic chromosome problem that some babies are born with. A normal baby inherits 23 chromosomes from the mother and 23 from the father. In Down syndrome, the baby gets an extra chromosome, a total of 47 chromosomes instead of 46. It is this extra genetic material that causes the physical features and retarded development associated with DS.

A baby with Down’s syndrome can usually be recognised after birth by the typical flat facial features with upwardly slanting eyes and a protruding tongue. The babies also appear floppy due to decreased muscle tone. They may have small ears and a single palmar crease.Baby

Anyone can have a baby with Down’s syndrome but a woman’s risk increases as she becomes older, rising from a 1 in 1,500 risk in a 20 year old to a 1 in 50 risk or greater in a 45 year old woman. About 3/4 of foetuses with Down’s syndrome never actually develop fully and miscarry. However, one quarter do mature to full term.

Down’s syndrome can be diagnosed both before birth (prenatally) as well as after birth (postnatally). After birth diagnosis is confirmed by genetic tests if the looks of the baby arouse suspicion. A blood test is conducted to look at the appearance and number of chromosomes (karyotype) of the baby to detect the extra chromosome.

All pregnant women should be offered a screening test for Down’s syndrome so that they can make an informed decision about their pregnancy. A screening test tells us only the ‘likelihood’ of the baby having Down syndrome.  It is important to understand this. The test does not give a definitive answer as to whether the baby has Down’s syndrome or not. If the screening test shows a high likelihood or risk the next step would be to do a diagnostic test.

There are two basic methods of screening for Down’s syndrome, the ultrasound scan and biochemical serum screening.

An ultrasound scan is carried out between 11 weeks and 2 days and 14 weeks and 1 day of pregnancy. It measures the fluid collection under the skin at the back of the baby’s neck (the nuchal translucency (NT).  Foetuses with Down syndrome tend to have more fluid in the back of the neck than normal foetuses. The measurement of the fluid layer in the back of the neck is called the nuchal translucency.Ultrasound

About 80% of Down syndrome fetuses will have a nuchal translucency measurement that is thicker than most normal fetuses. The scan alone cannot make a definite diagnosis of Down syndrome. Even if the scan is normal it does not completely exclude the possibility that a fetus might be affected.

Blood tests (also called biochemical serum screening) are often combined with the NT test.  Pregnancy associated plasma protein A (PAPP-A), beta human chorionic gonadotrophin (beta-hCG) and alpha-fetoprotein (AFP) are tested for. These hormones and proteins are produced by the placenta or the developing baby. If the baby has Down’s syndrome, the levels of these substances can be affected.

A computer program is used to give the risk of the baby having Down’s syndrome, based on the blood test results, the NT results, the size of the baby, the age, the stage of the pregnancy, the weight, ethnicity and smoking status of the woman.

The screening test will give the risk in this pregnancy of the baby being born with Down’s syndrome. For example, the test may show that there is a 1 in 1,000 risk of having a baby with Down’s syndrome. This is quite a low risk.

If the screening test results show a risk of between 1 in 2 to 1 in 150 that the baby has Down’s syndrome, this is taken as a higher risk result.

A higher risk result does not indicate definitely that the baby has Down’s syndrome. With a higher risk result, further tests are needed to confirm the diagnosis and give a definite ‘yes’ or ‘no’ answer.

The two main diagnostic tests that are used to diagnose Down’s syndrome before birth (prenatally) are amniocentesis and chorionic villus sampling (also called CVS).

Amniocentesis is usually safest after 15 completed weeks of pregnancy (most commonly between 15-18 weeks). During amniocentesis, a sample of the amniotic fluid inside the womb (uterus) that is surrounding the developing baby is taken using a fine needle. The castaway cells of the baby in this fluid can be examined for genetic material to diagnose Down’s as well as other genetic defects.

During Chorionic villus sampling or CVS,  a biopsy of tissue from a part of the placenta called the chorionic villi is taken. The cells of the chorionic villi contain the exact same genetic material (chromosomes and DNA) as the cells of the developing baby and this can be examined.

CVS is usually done between the 11th and 14th week of pregnancy. There is a small risk of miscarriage withCVS.

It is upto the woman to decide whether she wants to go in for diagnostic testing for Down’s syndrome or not.

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