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Although Down syndrome cannot be prevented, it can be detected

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Precision Medicine | Preconception Genetic Testing | Targeted Therapy for Cancers | Oncogenetic Screening


Pregnant women often pay attention to all kinds of possible issues because they want their baby to be as healthy as possible. One condition that they worry about is Down syndrome and we believe that no one wants their baby to face this problem. The world is still waiting for medical advances in Down syndrome treatment. Although Down syndrome cannot be prevented, the latest medical advances include screening tests which can estimate the chance that a baby may have Down syndrome. “World Down Syndrome Day is a global awareness day that is annually observed on March 21. Here is an update on what is currently known about Down syndrome.

What is Down syndrome?

Down syndrome: trisomy 21 is a genetic disorder caused by a chromosome anomaly. The human chromosome complement consists of 23 pairs of chromosomes – 22 pairs of autosomes and 1 pair of sex chromosomes, half from the father and half from the mother. Chromosomes usually come in matching pairs. However, a baby with Down syndrome will have 3 copies of the No. 21 chromosome. Down syndrome may occur in any pregnancy and has nothing to do with race or socioeconomic status. The date for World Down Syndrome Day is the 21st day of the 3rd month, which was selected to signify the uniqueness of the triplication of the 21stchromosome which causes Down syndrome.

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What does a child with Down syndrome look like?

A child with Down syndrome can be easily distinguished; there are two characteristics as follows:

  • Physical features: Of the obviously noticeable features, children with Down syndrome have a small, flat head. The eyes slant upward and the nose is flattened. The mouth is small, making the tongue appear large. All children with Down syndrome tend to look very similar.
  • Intellectual disabilities: Children with Down syndrome tend to have a lower than average intelligence quotient (IQ).

What are the risk factors for Down syndrome?

The risk factor for Down syndrome includes maternal age. The incidence of births of children with Down syndrome increases with maternal age. At the age of 25, a woman’s risk of conceiving a child with Down syndrome is about 1 in 1,500. By age 35, however, the risk is about 1 in 350. A woman’s chances of giving birth to a child with Down syndrome increases with age. Many people wonder whether the families are in some way involved in causing Down syndrome; a ‘yes’ answer is possible, but rare, so it is a small factor. Can Down syndrome be attributed to the father? The answer is also possible ‘yes’, but it is rare. As mentioned above, Down syndrome is a chromosomal disorder, but most cases of Down syndrome are not inherited. The key known risk factor for this disorder is maternal age during pregnancy. As a matter of fact, pregnant women at every age group are at risk. Many people think that only women above the age of 35 are at risk of having a baby with Down baby, which it is not true, because younger women are at risk too. These days, pregnancy rates are higher among teenagers. This means the number of young mothers is increasing in Thailand. In fact, the number of young mothers is higher than those of older ones. Although, older pregnant women are at higher risk of conceiving a child with Down syndrome, they are also more cautious and more likely to undergo screening tests. Younger pregnant women often think that they have no risk, so they do not need the screening tests. Even though the possibility of having a baby with Down syndrome is small amongst younger mothers, there is still a chance. Lack of screening may be a contributor for younger mothers; an increase in Down babies born to younger mothers has been observed.

Down syndrome and other health problems

Children with Down syndrome will not only have the same physical features and intellectual disabilities, but they are also at increased risk for other health conditions. They often have abnormalities in various organs as a result of of this syndrome. The most common conditions are heart defects and muscle development in children with Down syndrome is often slower. Additionally, they are at a higher risk for leukemia.

Are you at risk of having a baby with Down syndrome?

Although Down syndrome cannot be cured, advances in genetic testing enable us to estimate the chance that a baby in the womb may have Down syndrome. There are two types of tests for Down syndrome:

  1. Screening tests are used to estimate the likelihood that a mother-to-be is carrying a baby with Down syndrome from the following information:
  • Mother’s age during pregnancy and history of pregnancy: As mentioned above, every age group is at risk. However, as a woman gets older, her risk increases. For women who have had a first child with Down syndrome, the chance of having the second baby with Down syndrome increases.
  • Ultrasound: NT (nuchal translucency) is used to measure extra fluid at the back of the baby’s neck. If it is greater than the normal range of 3mm, the baby may have some defects, but this alone does not suggest that the baby will have Down syndrome. It can be caused by other defects, including heart disease and other genetic disorders. In this case, the Down syndrome screening test alone is not good enough, and gene chromosomal microarray analysis and fetal echocardiography are required. After the NT, we will check whether the baby’s organs are developing normally. If any abnormalities are found, we will perform the genes test using chromosomal microarray analysis.
  • Mother’s blood test: Biochemical markers and fetal cell-free DNA are measured in the mother’s blood. There are several types of tests that provide different levels of precision and accuracy. The integrated test offers the most effective method of biochemical screening and the detection rate for Down syndrome is 95%. The NIPT (non-invasive prenatal testing), which uses fetal cell-free DNA from the mother’s blood, has an accuracy rate of 99.3%. The NIPT has a low false positive rate and reduces the risk of unnecessary extraction of amniotic fluid – amniocentesis.

The first screening test is usually performed between 11 and 13 weeks, six days of your pregnancy, starting from the NT conducted within this period, not before or after it. This is why it is so important to start prenatal care as early as possible. In fact, the NIPT can be carried out from 10 weeks of pregnancy, if necessary. However, we usually perform the NIPT after 11 weeks because we will perform the NT and check for the baby’s abnormalities at the same time. If the NIPT is done at the 10-week stage, then the ultrasound has to be done at 11 weeks, and in case of any thickness of NT or other abnormalities being found, the collection of amniotic fluid is required for chromosomal microarray analysis. This means unnecessary NIPT costs are incurred.

  1. Diagnosis: The diagnostic tests can be compared to a biopsy in a cancer screening test. The tests can tell if the baby has Down syndrome or not. The diagnostic tests are performed based on the duration of pregnancy as follows:
  • Between 12-13 weeks of pregnancy: chorionic villus sampling (CVS) (placenta) is required
  • Between 15-20 weeks of pregnancy: amniocentesis (amniotic fluid) is required
  • About 20 weeks of pregnancy or longer: cordocentesis (fetal blood) is required

There are pros and cons to each test. The diagnostic tests can give a definite answer; however, the tests may increase the chances of miscarriage. The miscarriage rate for these tests is between 0.2 – 2%.

The screening and diagnostic tests are different in terms of accuracy and complications. Many women believe that these are the same, but they are not. Theoretically, pregnant women are not required to undergo both tests; however, a doctor should advise expectant mothers to help them make a decision on whether to undergo the screening or diagnostic tests. They can choose to have screening tests, followed by diagnostic tests, or choose to have diagnostic tests only.

In conclusion, although Down syndrome cannot be cured, the available tests can indicate the chances of an expectant mother having a baby with Down syndrome. If there is a chance of a baby being born with Down syndrome, the parents can plan their future family life and can decide whether to keep or terminate the pregnancy. It is always their decision. For the parents who choose to carry on pregnancy, even though they have certain disabilities, they can develop and live happily in society with family support. Do not forget that all pregnant women, at every age group are at risk to some extent, so it is important to get early tests to determine whether the baby has Down syndrome, so the parents can plan to deal with the issue in their chosen way beforehand.

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Related information:
Precision Medicine | Preconception Genetic Testing | Targeted Therapy for Cancers | Oncogenetic Screening

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Assoc. Prof. Boonsri Chanrachakul, M.D. Summary: Obstetrics And Gynaecology Maternal And Fetal Medicine