Prenatal Testing

Question 1

What percentage of babies born in Victoria have a birth defect?

Answer 1

4% (including stillbirths)

Question 2

Why is prenatal testing done?

Answer 2

allows couple the choice to terminate, allows couple to psychologically prepare, allows health care team to prepare, gives reassurance with a normal result

Question 3

What is a prenatal screening test?

Answer 3

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

Question 4

What is measured in the first trimester combined screening test?

Answer 4

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

Question 5

What is the cut-off for T21 for first trimester combined screening?

Answer 5

1 in 300

Question 6

What is the cut off for T18 for first trimester combined screening?

Answer 6

1 in 175

Question 7

What is the detection rate for T21 in first trimester combined screening?

Answer 7

93%

Question 8

What is measured in second trimester maternal serum screening test?

Answer 8

blood taken at 14-20 weeks which measures 4 biochemical analytes - beta hCG, inhibin, oestriol and alpha fetal protein

Question 9

In second trimester maternal serum screening what are the analytes results that give an increased risk for trisomy 21?

Answer 9

low alpha fetal protein and oestriol and high hCG and inhibin

Question 10

In second trimester maternal serum screening what are the analytes results that give an increased risk for neural tube defects?

Answer 10

raised alpha fetal protein

Question 11

What is the cut off for T21 for second trimester maternal serum screening?

Answer 11

1 in 250

Question 12

What is the cut off for T18 for second trimester maternal serum screening?

Answer 12

1 in 200

Question 13

What is the cut off for neural tube defects for second trimester maternal serum screening?

Answer 13

> 2 MoM

Question 14

What is the detection rate for T21 in second trimester maternal serum screening?

Answer 14

85%

Question 15

What is the detection rate for neural tube defects in second trimester maternal serum screening?

Answer 15

93%

Question 16

What is genetic carrier screening?

Answer 16

a test to see if parents are carriers for autosomal recessive conditions

Question 17

What women are offered invasive diagnostic prenatal testing?

Answer 17

women with an increased risk screening result, women of advanced maternal age, couples who are known carriers

Question 18

What are the two methods of diagnostic prenatal testing?

Answer 18

chorionic villus sampling and amniocentisis

Question 19

What is chorionic villus sampling?

Answer 19

a needle is inserted into placental tissue under ultrasound

Question 20

What is amniocentisis?

Answer 20

a needle is inserted under ultrasound and 20ml of amniotic fluid is collected

Question 21

What tests are done with the sample collected from CVS or amniocentisis?

Answer 21

FISHing and karyotyping and chromosomal microarray

Question 22

What is FISHing for chromosomes?

Answer 22

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

Question 23

Why does karyotyping take a long time?

Answer 23

because cells need to be grown in culture for 2 weeks

Question 24

What is reciprocal translocation?

Answer 24

exchange of material between two non identical chromosomes

Question 25

What is a Robertsonian translocation?

Answer 25

a translocation between the two long arms of two acrocentric chromosomes joined at or near the centromere (13,14,15,21 and 22)

Question 26

What percentage of Down's syndrome cases are due to an unbalanced translocation?

Answer 26

5% - of which some are due to inheritance from a parent with a balanced translocation

Question 27

What is a chromosomal microarray?

Answer 27

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

Question 28

What is pre implantation genetic diagnosis?

Answer 28

genetic testing (FISH and chromosomal microarrays) done on a 3 day old zygote before implantation for IVF

Question 29

What is non invasive pre natal screening?

Answer 29

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