Genetic testing Flashcards
In a ‘normal’ pregnancy describe what tests are carried out
1) positive pregnancy test (possibly at home)
2) Nuchal scan at 10 to 14 weeks (ideally at 12 weeks)
3) mid trimester anomaly scan at 20 to 22 weeks
What is nuchal translucency and how is it used as a screening test and when is it done
It measures the thickness of the fluid at the back of the baby’s neck. If it is greater than 3mm, suggests that there is an abnormality including:
1) chromosomal (Turners, Edwards, Downs, Pateau
2) birth defects (cardiac, renal and abdominal wall defects)
3) skeletal dysplasia
What type of test is a nuchal translucency
screening (not diagnostic)
When is a prenatal test arranged after a nuchal translucency (NT)
1) If thickness of fluid at back of baby’s neck is more than 3 mm
2) If any abnormalities found on the nuchal translucency
3) familial history of a disease
4) Previous pregnancies with Downs syndrome or Cystic Fibrosis
5) parents identified as having a genetic condition
What further tests could be carried out if NT is abnormal
1) fetal MRI
2) fetal cardiac scan
What are 4 major aims of genetic testing
1) inform parents of abnormalities
2) possible in utero treatment
3) treatment after birth
4) possible termination of fetus
What makes it possible for us to remove cell free fetal DNA (cffDNA)
fetus’s DNA is in the placenta and a v small amount of this enters the mothers blood
and you can analyse these DNA fragments
When can you measure cffDNA
present in the blood from 4 to 5 weeks but can only be reliably detected after 9 weeks
What does NPID stand for and does cffDNA help in this
NPID = non invasive pre natal diagnosis
and yes
What conditions can be tested for using cffDNA and at what time
1) At 9 weeks:
2) Achondroplasia
3) thanatophoric dysplasia
4) Apert’s Syndrome
5) Sexing can be done so that if the fetus
6) has the SRY gene
7) this indicates that they are male
8) and further invasive testing is needed if
9) there is a familial history of a sex linked condition
What other conditions can be tested for using NIPD by using cffDNA methods
1) autosomally dominant single gene mutations or de novo gene mutations (eg NF1)
2) for autosomally recessive conditions where the paternal alteration has been inherited
What would you do if cffDNA detects that for an autosomal recessive condition, the paternal alteration has been inherited
do an invasive test to see if the maternal alteration has also been inherited
What is special about the diagnosis of CF when using cffDNA
you can use haplotyping so there would be no need to have a further invasive test (only available for CF now)
