Genetic Screening

PRE-NATAL TESTS

A Syndrome is defined as a series of different symptoms and characteristics derived by different causes. Down’s Syndrome, in particular, can’t be prevented but it can be identified before a child is born. All pregnant women, independent of their specific age, are offered genetics tests to determine their risks of having a child with a health disorder (4). There are two types of genetics prenatal tests for Down’s Syndrome: screening tests and diagnostic tests.

Screening tests are non-invasive tests that can only estimate the chance of having a child with the Syndrome. This means that the test does not give a definite response on whether or not the baby has Down’s Syndrome, but, if there is a high risk indicated in the result, the doctors will recommend diagnostic tests. Screening tests involve a blood test and an ultrasound scan, also called combined tests. (3)(4)

Diagnostic tests are invasive tests that give a definite diagnosis with almost 100% accuracy (3). This type of test has a very low risk of complication which may lead to miscarriage (spontaneous termination).  Diagnostic tests involve a Chorionic Villus Sampling (CVS) and an Amniocentesis.

 

NON-INVASIVE SCREENING TESTS

ULTRASONOGRAPHY:   In the first trimester of pregnancy (10-14 weeks) (4), women are offered an ultrasound exam. During the test  the researcher looks for typical features that characterize foetuses with Down’s Syndrome. These include nuclear translucency and analyses of foetal nasal bone. Nuclear translucency  is a collection of fluid beneath the skin of foetal neck. Normally it is less than 2.5-3mm (1) , if this value increases , it could indicate Down’s Syndrome.  The analyses of foetal nasal bone should show three distinct lines, that testify nasal bone ossification (6). Both the nasal bone and the nuclear translucency  are not always visible in relation to the baby’s position, so sometimes the exam should be repeated later, or further blood tests are added.

BLOOD TESTS: Blood tests, or serum screening tests, can be carried out in the first (10-14 weeks) and in the second (14-20weeks)trimester of the pregnancy. The researchers, analysing the blood sample, look for foetal chromosomal material in the mother’s blood  and measure the amount of certain substances that may be related to a foetus with Down’s Syndrome (4).

 

INVASIVE DIAGNOSTIC TESTS

CORONIC VILLUS SAMPLING:  Is a test carried out in the first trimester of pregnancy (10-11 weeks). During CVS a small sample of tissue  from placental chorionic villus is taken inserting a needle in the abdomen or through the cervix of the mother. Cells of the chorionic villus contain the same genetic material of foetal  cells. It is very accurate, however, it has a 1-1.5% risk of miscarriage (2).

AMNIOCENTISIS: Is performed after 14 weeks of pregnancy ( 15-17 weeks). During Amniocentesis a sample of 20-30ml of amniotic fluid is withdrawn from the abdomen of the mother. The procedure requires the help of ultrasonography to locate the placenta and the baby, such that a needle can be confidently inserted. The sample will contain foetal cells to be analysed for Down’s Syndrome markers. As for the CVS, this test is very accurate but it carries a 1% risk of miscarriage(2).

 

NATAL/AT BIRTH TESTS

An accurate diagnosis for Down’s Syndrome can also be made with at-birth tests. The new born baby will be examined both for his physical and genetic features.

As previously explained, babies with Down’s Syndrome are identified by some typical characteristics, but those can be present also in babies without the Syndrome (3), in this cases a Karyotype analyses is performed.

To analyse the Karyotype of a baby means that a blood sample is withdrawn and chromosomes are extracted, photographed and organized by size, number and shape (3). In this way the researcher can definitely confirm  the diagnosis for Down’s Syndrome.

 

 

REASONS TO DO THE TEST: It is not mandatory to do the tests. You can freely decide to have tests for birth defects or not. However, several factors must be considered:  maternal age, your need for information, your personal beliefs and values and what level of risk you are willing to take in order to obtain information. You have to think if having a precise diagnosis will help you or not in making decisions and if it will help you with coping a positive result.

The main aim of prenatal diagnosis is to promote informed reproductive decision making (5). Prenatal testing and genetic counselling have shown to help individuals and couples  to understand the diagnosis and the possible options. This means that having information will help you to choose the course of action to undertake.

The tests provide information for all couples, for those  for whom pregnancy termination is an option and for those that will continue the pregnancy anyway.  It helps clinicians and new parents to make plans for the health care of the child diagnosed with the syndrome (2). In this way  the doctor can answer nearly all the possible questions.

In this way families can be prepared both  practically and psychologically.

 

LIMITATIONS OF THE TESTS: Genetics tests are not 100% accurate(2), so you have to evaluate the benefits and the degree of certainty of the tests and decide if they are worth the risk.

For some couples learning of genetic risk factors can create anxiety and stress, that  is exacerbated by the need of a rapid decision (5).

Furthermore, each child with Down’s Syndrome have different degrees of symptoms, but the tests won’t tell you how much a child is affected, but only if he has the mutation or not.

There are also several ethical issues to be considered. Not everybody agrees with the increased use of prenatal testing. Disability right claims that positive results often lead to pregnancy termination (5), and this leads to lots of social and ethical questions.

On the one hand, each women must have reproductive freedom but, on the other hand,” families may sometimes decide to abort a wanted pregnancy, basing their decision on one trait of the child (i.e. Down’s Syndrome) rather than seeing the foetus as a future child with many traits” (5).

People “ fears that this will lead to more discrimination and stigmatization against children with Down’s Syndrome and intolerance for those parents who decide to continue a pregnancy of a child with the disability” (5). Furthermore, parents with children with Down’s Syndrome fear that the advent of always new genetic tests will lead to the termination of research for a cure and that no more children with Down’s Syndrome will be given birth to.

Article by: Francesca Picco

References

  1. Hovsepian, S., Sharifzadeh, M., Adibi, A. and Kazemi, K. (2015). Normal reference range of fetal nuchal translucency thickness in pregnant women in the first trimester, one center study. Journal of Research in Medical Sciences, 20(10), p.969.
  2. Jorde, L., Carey, J. and Bamshad, M. (n.d.). Medical genetics. 3rd ed. Mosby, pp.278-315.
  3. NDSS. (2017). What is Down Syndrome? | National Down Syndrome Society. [online] Available at: http://www.ndss.org/about-down-syndrome/down-syndrome/ [Accessed 8 Dec. 2017].
  4. Nhs.uk. (2017). Screening for Down’s, Edwards’ and Patau’s syndromes in pregnancy – Pregnancy and baby guide – NHS Choices. [online] Available at: https://www.nhs.uk/conditions/pregnancy-and-baby/pages/screening-amniocentesis-downs-syndrome.aspx [Accessed 8 Dec. 2017].
  5. Singer, P. and Viens, A. (2008). The Cambridge textbook of Bioethics. Cambridge University Press, pp.104-108.
  6. Viora E, e. (2017). Ultrasound evaluation of foetal nasal bone at 11 to 14 weeks in a consecutive series of 1906 foetuses. – PubMed – NCBI. [online] Ncbi.nlm.nih.gov. Available at: https://www.ncbi.nlm.nih.gov/pubmed/14558019 [Accessed 8 Dec. 2017].