Genetic Testing Flashcards
When is a nuchal scan done?
10-14 weeks gestation
What 5 things can a Nuchal Scan do?
- date the pregnancy
- diagnose multiple pregnancies
- disgnose major fetal abnormalities
- diagnose early miscarriage
- assess the risk of down syndrome
What technique is used to assess the risk of down syndrome?
Nuchal translucency scan
At how many weeks is a mid trimester scan done?
20-22 weeks
What is the purpose of the mid trimester anomaly scan?
To take a more detailed look at specific organs like heart, brain, spinal cord and to check for 11 rare conditions
What are the two conditions under which prenatal testing is arranged?
If the nuchal scan or mid trimester scan brings up an increased risk of down syndrome, or parents are carriers for a specific condition
What is measured during a Nuchal Translucency test?
The thickness of fluid at the back of the foetal neck
A nuchal translucency if more than how many mm indicates a problem?
anything more than 3mm
If the nuchal thickness is >3mm, what 3 things can this indicate?
- Increased risk of down syndrome
- Birth defects
- Chromosomal abnormalities
What birth defects can arise from an increase nuchal translucency?
- Cardiac abnormalities
- Pulmonary abnormalities
- Renal defects
- Abdominal wall defects
- Skeletal dysplasias
What is linked to nuchal translucency which increases the risk of the baby having down synrdome?
Maternal age
What type of test is a nuchal translucency scan?
A screening test
What are the 4 main aims of prenatal testing?
- Inform / prepare parents
- Allow for the termination of pregnancy if necessary
- Help manage the remainder of the pregnancy
- Allow possible in utero treatment
What type of scan is used to look at the fetal organs in more detail?
A fetal MRI
What serum markers are tested in maternal serum screening?
Serum markers for trisomy 18, 21 and neural tube defects
How do serum markers in the mothers blood for conditions like neural tube defects get there?
Leak from placenta into mothers blood
When is the 1st trimester maternal serum screening and NT measurement done at?
11-14 weeks
What is PAPP A?
Hormone made by placenta during pregnancy
What might a low PAPP A suggest?
Defective / smaller placenta
When are PAPP A levels measured?
11-14 weeks
What is measured during the 2nd trimester maternal serum screen?
AFP, UE3 and hCG
What does cell free foetal DNA screening do?
Analyses the placental DNA in maternal plasma
When is cffDNA first ever detectable?
4-5 weeks gestation
When is the Cell-Free Foetal DNA reading most accurate?
9 weeks
What three conditions can you be pre-natally tested for for free on the NHS?
- Apert Syndrome
- Achondroplasia
- Thanatophoric dysplasia
What gene is detected on the Y chromosome in sexting tests?
The SRY gene = males
In what instance is sexing offered?
when there is an X-linked condition in the family - males more at risk so if the baby is male, then go on to pre natal tests to double check
What are the two types of invasive tests required?
Amniocentesis and Chorionic Villus Sampling
Where are the chorionic villi found?
As part of the developing placenta
Why is a sample of the chorionic villi taken?
It has the same DNA as foetus
Why is CVS chosen over amniocentesis?
It allows the parents to have their results earlier than amnio
When is CVS done?
11-14 weeks, usually at 12
How are CVS samples taken?
Transvaginally or transabdominally
At what stage is amniocentesis done at?
from 16 weeks
What does amniocentesis involve?
Sample of amniotic fluid is taken as it contains foetal cells
What are the 3 risks associated with Amniocentesis?
- Rh sensitisation
- Miscarriage
- Infection
What happens to cffDNA in women with a higher BMI?
Lower cffDNA count as they have more of their own DNA
What is the limitation of non-invasive prenatal tests involving twins?
not possible to tell which foetus the DNA is from
What are the benefits of non-invasive pre-natal testing over invasive techniques
- No risk of miscarriage
- Less impactful on the baby / mother
- Less expertise is needed to perform the tests
- Can be done earlier than invasive techiques
What is the purpose of a CGH array?
looks for imbalances in chromosomes that may lead to diseases
What may be done after a CGH array if chromosomal imbalances are found?
Testing of parents to see if they are carriers
What is a trio exome test?
A full genetic sequencing
When are trio exome tests offered?
When previous pregnancies showed serious anomalies eg brain, skeletal and heart
What are the four potential options for reproduction?
Conceive naturally
Adopt
Use egg / sperm donors
Pre-Implantation Genetic Diagnosis - IVF
when is pre-implantation genetic diagnosis used?
For conditions caused by single faulty gene to reduce the amount of non-embryo DNA (including sperm DNA) which could make the risk of a wrong diagnosis higher
At what stage is the embryo biopsied?
8 cell stage
What is an intracytoplasmic sperm injection?
When a single sperm is injected into the center of each egg
What are the downsides of PIGD?
- Can be emotionally stressful
- Lengthy and expensive process
- Success rate is low - around 30% per cycle
How many round of PIGD are couples usually funded for provided they fit the criteria?
3
What is the role of a genetic counsellor in prenatal testing?
- Arrange CVS and amnio
- Provide emotional support for parents
- Provide information on termination options
- Facilitate decision making
- give results of tests
- Discuss recurrence risks and plans for future pregnancies
