18.05.01 Prenatal sampling and considerations Flashcards
What are some properties of cell foetal DNA?
1) cffDNA is foetal DNA circulating freely in the maternal blood stream
2) Can be obtained through non-invasive sampling
3) Appears in the maternal blood from ~5/40
4) Present in 200bp fragments (this is smaller than mat. DNA which allows for differentiation)
5) Derived from apoptotic trophoblasts which make up the placenta.
6) 2-6% of maternal plasma. Some studies suggest 10-20%
What is the goal of prenatal testing?
To allow expectant couples to make an informed choice regarding their pregnancy
What factors will influence the decision of what/if prenatal sample type to take?
1) Referral reason
2) Gestation
3) Amount of material
4) Risk to foetal and maternal health
5) Rapidity and accuracy of genetic results
What is CVS material? When in pregnancy it is usually obtained and how?
Placental tissue made of trophoblasts and mesenchyme cells.
11+0 - 13+6 / 40
Typically 10-25mg obtained either transabdominally or transcervically.
What are the risks associated with CVS sampling?
1-2% risk of miscarriage
~1.5-3% will show CPM
Maternal cell contamination possible
What is amniotic fluid? When in pregnancy it is usually obtained and how?
Foetus is surrounded by the amnion - fluid filled sac - which acts as a protective barrier, maintains temperature and promotes symmetrical growth and lung development.
Usually taken from 15 weeks in pregnancy
UP to 20ml sample taken transabdominally
What are the risks associated with AF sampling?
0.5-1% miscarriage
Can lead to infection and placental/foetal damage.
Samples taken at earlier gestation have higher culture fail rate, higher miscarriage risk and greater number of extra-embryonic cell growth (rather than foetal)
What is the composition of amniotic fluid?
Fluid is heterogenous mix of shed foetal cells e.g. from lung, respiratory tract and skin
65& extra-embryonic membranes and amniocytes from the amnion
25% epitherlial
15% fibroblasts
Gives a more accurate representation of the foetal than CVS
When can foetal blood samples be taken? Why might this be the preferential sample type?
18-20/40
Performed when veins are developed by insertion of a needle into the umbilical cord (aka - percutanenous umbilical cord blood sampling, cordocentesis)
Used when other techniques are inconclusive (e.g. mosaic) and a result is required rapidly.
Commonly used for suspected blood disorders e.g. foetal anaemia, immunology or deminse
What are the risk of FBS?
Risk of foetal loss is ~2%
High risk of complication so only used in pregnancies with a very high risk of genetic defects
What other sample types might be used to determine the foetal karyotype?
1) POC
2) Skin
3) Pleural effusion
What factors will influence the level of MCC present in a sample?
1) Sampling technique
2) Sample quality
3) Operator
4) Method of sample processing
Why is the rate of MCC significantly lower in cultured AF samples than in direct AF samples?
Culture conditions favour the growth of amniocytes and reduce/eliminate blood cells.
However culture AF more likely to represent a homogenous cell type reflecting pure foetal origin.
Culture AF more likely to represent a homogenous cell type reflecting pure foetal origin that direct AF samples, why?
Maternal blood cells senesce during culture, however prolonged culture may allow overgrowth of maternal fibroblasts and epithelial cells (potentially leading to MCC).
Discarding the first draw can reduce this possibility.
How is the possibility of MCC reduced in CVS and POC culture?
By careful separation of the maternal decidua from chorionic villis before culture initiation.