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Flashcards in Prenatal testing Deck (32):

How many pregnancies are affected by neural tube defects? How can these be prevented?





1/500, can prevent with folate supplements



Why is reproductive testing utilised?

Offers reproductive choices

Provides risk information to couples who may otherwise not choose to begin a pregnancy

Choice of termination of affected foetus

Allows couples to prepare for affected baby

Helps medical team to prepare for delivery, management and care of affected baby

Provides reassurance when result is normal




What types of prenatal testing are available?

SCREENING TESTS: all pregnant women, non-invasive to foetus

DIAGNOSTIC TESTS: for women at increased risk, invasive to foetus



What is the purpose of prenatal screening?

Identifies a subset of women from the general population that are at an increased risk of having a baby with a birth defect


Are prenatal screening tests definitive?

No - need to follow up with diagnostic tests


Which prenatal screening tests are used in Victoria? What does each detect?

1st trimester combined screening: T21, T28

2nd trimester combined screening: T21, T18, NTD 


What is involved in first trimester combined screening?

Blood at 9 weeks - 13 weeks and 6 days
Measure PAPP-A and bHCG

Us at 11 weeks and 3 days - 13 weeks and 6 days
Measure nuchal translucency and crown rump length (dating)




What is nuchal translucency?

Fluid behind back of baby's neck

Normal, but abnormal for it to be very thick

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How is a risk calculated after 1st trimester combined screening?

What are the cut-offs?

What is the detection rate for T21 wuth this testing?

Combine results of blood analysis, US and other factors (e.g. maternal age)

T21: 1 in 300
T18: 1 in 175

T21 detection rate is 93%



What is involved in 2nd trimester screening?

Blood taken at 14-20 weeks
Measures PAPP-A, bHCG, inhbin-A and a-fetoprotein


How is a risk calculated after second trimester screening?

What are the risk cut-offs?

What is the detection rate for T21 and NTDs?

Result of blood tests plus other risk factors (e.g. maternal age)

T21: 1 in 250
T18: 1 in 200
NTD: 2.0 MoM

Detection rate for T21 is 85%, NTDs in 93%




What sort of testing can be carried out before pregnancy?

Genetic carrier screening 


What are the options after an increased risk is detected in first or second trimester screening?

No action


Foetal anomaly US with/or CVS or amniocentesis


Which women are offered CVS and amniocentesis?

Increased risk screening result

Advanced maternal age

Known carriers


Which tests can be carried out on the cells collected via CVS or amniocentesis?




Describe CVS?

Chorionic villus sampling 

From 11 weeks gestation

Sample placental tissue


Describe amniocentesis?

From 15-16 weeks

Sample amniotic fluid containing sloughed off foetal cells 


Describe the risk of miscarriage associated with CVS and amniocentesis?

CVS: 1% above background risk

Amniocentesis: 0.5% above background risk


Describe the FISH technique?

Fluorescent probes on chromosomes in non-dividing cells

Count number of chromosomes 


How is a karyotype obtained?

Cells grown for 2 weeks
Arrested in metaphase
Then chromosomes released to form karyotype


What types of rearrangements can occur between chromosomes?

Reciprocal translocation: part of one chromosome attaches to another (balanced)

Robertsonian translocation: translocation between two acrocentric chromosomes (13, 14, 15, 21, 22)

Unbalanced translocation: extra copy of chromosome attaches to another chromosome 


Describe how Down syndrome can be inherited due to an unbalanced translocation?

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What can be detected using microarrays?

DNA sequences and/or gene expression



Number of chromosomes 


What are chromosomal microarrays used for?

Probes along entire genome

Detect copy number variations (deletions and microdeletions)

First tier test for developmental delay, intellectual diasbility, autism


Briefly describe how a chromosomal microarray is performed and interpreted?

Probes along genome

Loom to see if there are equivalent amounts of DNA between patient and sample 

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Which chromosomal abnormality is not detected by chromosomal microarrays?

Balanced translocations 


When should prenatal molecular karyotyping be performed?

Foetal abnormality detected on US

Nuchal translucency > 3.5mm

Karyotype identifies a complex change

Family member has a microdeletion syndrome


What is NIPT/S?

Non-invasive prenatal testing/screening

Samples cell free foetal DNA/RNA in maternal blood 


When can NIPT/S be performed?

10 weeks gestation


How is NIPT/S performed?

Using massively parallel sequencing

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Is NIPT/S a screening or diagnostic test?

Very high sensitivity and specificity for trisomy 21 

Still recommend that positive result is followed up with CVS or amniocentesis


What is the disadvantage to NIPT/S?