Genetic testing Flashcards
What tests are done when there is a normal pregnancy?
Nuchal scan 10-14 weeks into gestation
Mid-trimester anomaly scan 20-22 weeks
( both are ultrasounds )
What are the clinical features looked at during the Nuchal scan and what can they mean?
Fluid behind foetal neck
Increases of > 3mm can indicate:
- Chromosome abnormalities (e.g. Down syndrome)
- NT + maternal age detects up to 75% of Down syndrome
- Birth defects e.g. cardiac anomalies, pulmonary defects (diaphragmatic hernia)
renal defects
abdominal wall defects
skeletal dysplasias
When is pre-natal testing arranged?
Following: abnormal findings of ultrasounds
which give an increased risk of Down Syndrome
• If: previous pregnancy affected with a condition e.g. DS, CF or parents are carriers of
chromosome rearrangement or genetic condition, e.g. DMD
What prenatal test involves scanning?
( ultrasounds and MRI )
- Nuchal translucency (NT) & nasal bone
- High level/anomaly scan
- Foetal MRI, at 20+ weeks
- Foetal Cardiac scan ( flow of blood )
How is prenatal non-invasive testing carried out? [ NIPD ]
Tests maternal serum markers in blood - ( 10-20%) DNA will be the baby’s - will be found in the mother’s blood from placenta.
What is cffDNA?
Cell free foetal DNA detectable from 4-5 weeks
Short DNA fragments
How to use NIPD to test for Down’s syndrome?
In trisomy 21 the amount of cfDNA for chromosome 21 is higher than in normal pregnancies
Which other NIPD tests are available for free on the NHS?
Achondroplasia
Thanatophoric Dysplasia
Apert Syndrome
What is Sexing used for?
Offered when there is an X-linked condition in the family e.g. DMD
Test detects SRY gene on Y chromosome
If fetus is male prenatal invasive testing will be carried out
What is haplotyping?
NIPD offered privately for cystic fibrosis to test for both maternal and paternal mutation
Is there cffDNA testing for Aneuploidy?
NIPT ( not diagnostic )
Privately offered
Tests for T13, T18, T21
What are the limitations of NIPD / NIPT ?
Multiple pregnancies - not possible to tell which fetus the DNA is from when having twins
The relative proportion of cell free fetal DNA is less in women with high BMI as they have their own cell free DNA
An invasive test may still be required to confirm an abnormal result ( NIPT )
What are the benefits of NIPD / NIPT ?
- number of invasive tests carried out will be reduced
- no increased risk of miscarriage
- less expertise is required to do blood test
- can offer NIPD/NIPT earlier than traditional invasive testing
What invasive tests are available and why may they be offered?
Offered if there is a known risk
- Chorionic villus sampling - CVS
- Amniocentesis
What is CVS?
Chorionic villus sampling:
- 11-14 weeks
- 1-2% miscarriage
- transabdominal or transvaginal
- Takes sample of Chorionic villi - part of developing placenta as it has same DNA as fetus
- allows an earlier result than amnio
