Testing for genetic disorders

Question 1

What is the main difference between the two types of prenatal tests offered - diagnostic and screening?

Answer 1

Diagnostic
- determines whether a baby will or will not develop a particular problem or condition after birth

Screening
- determines whether a baby has an increased risk of developing a particular problem or condition after birth. This is mainly just done for reassurance of the mother.

Question 2

When would you consider doing a prenatal diagnostic test?

Answer 2

It all comes down to evidence of the condition in the family:

• if the parents are both carriers for a condition
• if there is a strong family history of a condition
• if a previous child had been born affected
• if a parent has a condition

Other reasons:

• if the mother is of an increased maternal age
• if a screening test comes back with the baby having an increased risk of a condition
• if the baby has been exposed to some toxic/harsh chemical or environmental agent

-

Question 3

What are the main types of prenatal screening tests available and what is a common feature about all of them?

Answer 3

All non-invasive and pose no risk to the unborn foetus.

1. Ultrasound
   Usually done early on in the pregnancy to check the date of the pregnancy and how many babies are present. Further on in the pregnancy it may be able to identify physical abnormalities.
2. Nuchal translucency ultrasound
   This is performed in the first trimester and gives a measure of the depth of the fluid in the space at the back of the babys neck, This can be used to calculate the risk of the baby having downs syndrome.
3. Maternal serum testing
   This is simply a blood test of the mother by screening for several proteins it is used to calculate the risk of the foetus having problems such as downs syndrome.

Question 4

What are the main methods of prenatal diagnostic testing and what is common about all of them?

Answer 4

They are invasive tests that come with a small risk of miscarriage (

Question 5

Once the cells are extracted from an amniocentesis or chorionic villus sampling, what are the two main ways in which the DNA is analyzed?

Answer 5

FISH (fluorescent in situ hybridisation)
- to look for chromosomal abnormalities

DNA analysis
- via PCR (polymerase chain reaction) to amplify the DNA and then there is direct mutation analysis if there is a known family mutation/location or else indirect analysis using a DNA probe/marker

Question 6

What are the steps in offering testing to pregnant women?

Answer 6

all women - offer prenatal screening eg ultrasound

high risk women - offer non invasive prenatal testing eg maternal serum testing

If the screening tests are positive - offer prenatal diagnostic testing

Question 7

Why is genetic counselling important throughout these prenatal tests?

Answer 7

It can provide up to date information about the tests offered.

It ensures that parents have an informed decision about prenatal testing - espeshially diagnostic testing.

It is important that parents are able to discuss the advantages and disadvantages of testing and if tests reveal an increased risk of a condition then its its important that parents are able to discuss further diagnostic testing and prepare for what the future holds.