Prenatal Testing Flashcards
What percentage of babies born in Victoria have a birth defect?
4% (including stillbirths)
Why is prenatal testing done?
allows couple the choice to terminate, allows couple to psychologically prepare, allows health care team to prepare, gives reassurance with a normal result
What is a prenatal screening test?
a test which identifies which women are at risk of having a child with birth defect - the tests are non invasive and are not definitive
What is measured in the first trimester combined screening test?
blood taken at 10 weeks which measures PAPP-A and the beta subunit of hCG and an ultrasound to determine nuchal translucency, nasal bone and crown rump length
What is the cut-off for T21 for first trimester combined screening?
1 in 300
What is the cut off for T18 for first trimester combined screening?
1 in 175
What is the detection rate for T21 in first trimester combined screening?
93%
What is measured in second trimester maternal serum screening test?
blood taken at 14-20 weeks which measures 4 biochemical analytes - beta hCG, inhibin, oestriol and alpha fetal protein
In second trimester maternal serum screening what are the analytes results that give an increased risk for trisomy 21?
low alpha fetal protein and oestriol and high hCG and inhibin
In second trimester maternal serum screening what are the analytes results that give an increased risk for neural tube defects?
raised alpha fetal protein
What is the cut off for T21 for second trimester maternal serum screening?
1 in 250
What is the cut off for T18 for second trimester maternal serum screening?
1 in 200
What is the cut off for neural tube defects for second trimester maternal serum screening?
> 2 MoM
What is the detection rate for T21 in second trimester maternal serum screening?
85%
What is the detection rate for neural tube defects in second trimester maternal serum screening?
93%
What is genetic carrier screening?
a test to see if parents are carriers for autosomal recessive conditions
What women are offered invasive diagnostic prenatal testing?
women with an increased risk screening result, women of advanced maternal age, couples who are known carriers
What are the two methods of diagnostic prenatal testing?
chorionic villus sampling and amniocentisis
What is chorionic villus sampling?
a needle is inserted into placental tissue under ultrasound
What is amniocentisis?
a needle is inserted under ultrasound and 20ml of amniotic fluid is collected
What tests are done with the sample collected from CVS or amniocentisis?
FISHing and karyotyping and chromosomal microarray
What is FISHing for chromosomes?
fluorescent in situ hybridisation - fluorescent probes are attached to chromosomes to count the number of chromosomes - 3 colour FISH is X,Y and 21 and 5 colour FISH is X, Y, 21, 18 and 13
Why does karyotyping take a long time?
because cells need to be grown in culture for 2 weeks
What is reciprocal translocation?
exchange of material between two non identical chromosomes
What is a Robertsonian translocation?
a translocation between the two long arms of two acrocentric chromosomes joined at or near the centromere (13,14,15,21 and 22)
What percentage of Down’s syndrome cases are due to an unbalanced translocation?
5% - of which some are due to inheritance from a parent with a balanced translocation
What is a chromosomal microarray?
a genetic test which involves probes along the chromosomes and then hybridisation and by comparing copy number variations it can be determined whether there may be a gain or loss of genetic material - although wont detect a balanced translocation
What is pre implantation genetic diagnosis?
genetic testing (FISH and chromosomal microarrays) done on a 3 day old zygote before implantation for IVF
What is non invasive pre natal screening?
tests the cell free fetal DNA or RNA in the maternal blood for genetic abnormalities using MPS - still not officially diagnostic and would have to be followed up with a diagnostic test
