Antenatal Diagnosis of Fetal Abnormalities

Question 1

What % of neonates have a significant congenital abnormality?

Answer 1

2-3%

Question 2

What does the 20 week fetal anomaly scan detect?

Answer 2

• Abnormalities not compatible with life
• Associated with high morbidity or long term disability
• Foetal condition with the potential for intrauterine therapy
• Foetal conditions that will require special postnatal investigations or treatments

Question 3

What is chorionic villus sampling (CVS)?

Answer 3

Sticking a needle into and taking a biopsy of the placenta

Question 4

Why can CVS be used to for genetic diagnosis?

Answer 4

• Chronic (placental) villi are fetal in origin derived from the outer layer of the blastocyst (i.e. trophoblast) so share the same chromosomes
• Can also do metabolic investigations
• Can be transabdominal or transvaginal

Question 5

When can CVS be done?

Answer 5

After 11 weeks (11-14 weeks)

Question 6

What are the benefits of CVS?

Answer 6

1) Gives good preparations of DNA without culture
2) Chromosome analysis can be done rapidly within 24h however best quality chromosome analysis after culture (2 weeks)
3) Sampling extra-embryonic tissue
4) Earlier sampling and diagnosis than amniocentesis
5) Possibility of surgical termination in first trimester

Question 7

What are the risks/cons of CVS?

Answer 7

1) Higher risk of miscarriage 1-2%
2) Possibility of confined placental mosaicism - small risk that the cell line in the placental sample does not represent the fetus giving a FP or FN result
3) Sometimes not possible if the placenta is too posterior to access

Question 8

What is amniocentesis?

Answer 8

Sample from the amniotic fluid

Question 9

How is amniocentesis carried out?

Answer 9

• 10-20ml of amniotic fluid sampled by needle aspiration transabdominally under US guidance
• Mainly samples fetal epithelial cells
• Cells from amnion, fetal skin, urinary tract pelleted
• Cultured for 2 weeks to get sufficient numbers to analyse
• Longer culture may be needed for DNA or biochemical studies
• Rapid trisomy screen results by PCR bypass this time requirement

Question 10

Where does the needle go via in CVS and amniocentesis?

Answer 10

Vagina or abdomen

Question 11

What is the benefit of amniocentesis

Answer 11

Lower risk of miscarriage (0.5-1%)

Question 12

When is amniocentesis done?

Answer 12

14/15-16 weeks (often performed if fetal abnormality detected at 20 week scan)

Question 13

What are the cons of amniocentesis?

Answer 13

1) Fetal material
2) Later diagnosis bc have to wait longer to do test
3) Some loss of abnormal pregnancies
4) Occasionally the concentration of DNA present in the sample can be low, compromising the quality of the results
5) Sometimes the sample can be contaminated by maternal blood which can delay results in a small number of cases

Question 14

How may a post mortem be used to establish likely diagnosis?

Answer 14

• May be essential to establish likely diagnosis - closure, recurrence risk of happening again and offer test to parents
• Full post mortem
• Limited post mortem - photos, external examination, imaging
• Samples for chromosomes and DNA storage

Question 15

How is pre-implantation genetic diagnosis used?

Answer 15

• Done if know that fetus is at risk bc of parents
• IVF - embryos tested very early on for specific genetic disorder (at 5 days)
• Only unaffected embryos are re-implanted into the woman to try and achieve healthy pregnancy
• Available for serious genetic disorders on NHS - more cost effective than if baby with condition is born

Question 16

What other genetic diagnostic testing is offered in pregnancy?

Answer 16

1) Testing for single gene disorders (CVS/amniocentesis)
2) Parental chromosome rearrangement
3) Genome testing following detection of fetal abnormality

Question 17

What are indicating for offering prenatal diagnostic genetic testing (prenatal referrals)?

Answer 17

1) High risk screening result < 1:150
2) Fetal abnormality detected at 12 or 20 weeks scan
3) FH e.g. previous pregnancy/child with T21, carrier of chromosome rearrangement or carrier of single gene disorder
4) High maternal anxiety

Question 18

When is testing for single gene disorders carried out via CVS/amniocentesis?

Answer 18

1) Families with a child or previous fetus with a genetic diagnosis of a serious known condition e.g. spinal muscular atrophy, CF, DMD, sickle cell disease
2) Parents may be carriers
3) Parents may wish to test due to the gonadal mosaicism risk
4) If e.g. fetus has heart defect and NT but not trisomy can test for other causes

Question 19

Describe testing when there is a parental chromosome rearrangement

Answer 19

• One of the parents is known to carry a chromosome rearrangement that puts them at risk of having a child with an uxnabalcned chromosome complement e.g. T21 or T13

Question 20

How is genome testing done following the detection of a fetal abnormality on US?

Answer 20

1) Array CGH testing is carried out to detect the loss or gain of material across the genome
2) Fetal whole exome sequencing reports on pathogenic variants that the likely explanation of the fetal anomalies (not benign or unknown significance variants)

Question 21

What are the two types of genetic testing done that aren’t CVS or amniocentesis?

Answer 21

1) Antenatal CGH array

2) Fetal whole exome sequencing

Question 22

What does antenatal CGH array test for?

Answer 22

1) Significant deletions and duplications

2) Parental DNA tested for imbalance

Question 23

What do all types of genetic diagnostic testing test for?

Answer 23

1) Whole chromosome aneuploidy
2) Chromsome abnormality
3) Genetic imbalance
4) Monogenic disorders

Question 24

How are samples collected from invasive methods tested?

Answer 24

1) QF-PCR (rapid prenatal diagnosis) for aneuploidy detection ± sexing - microsatellite (repetitive DNA sequences) markers used to identify and count chromosomes 13, 18, 21 and X/Y
2) Array CGH to detect genomic imbalance
3) G-banded chromosome analysis
4) FISH in metaphase for confirmation of CGH
5) FISH in interphase to detect unbalanced products of a known familial balanced rearrangement
6) Mutation testing or sequencing to detect monogenic disorders

Question 25

Describe the features of rapid prenatal diagnosis

Answer 25

• Currently ~60-70% of invasive tests are carried out solely bc screening parameters indicate an increased risk of all 3 chromosomal trisomies
• Rapid testing using molecular cytogenic techniques can exclude or confirm trisomy within 24-48h
• Rapid testing is carrier out on all prenatal samples unless declined
• An abnormal rapid test result allowed rapid intervention and improved pregnancy management
• A normal rapid test result gives faster patient reassurance with minimal residual anxiety

Question 26

Explain how array CGH analysis is used as part of prenatal genetic diagnosis

Answer 26

• Genome-wide testing for chromosome imbalance
• Prenatal referrals with US abnormality are tested first by QF-PCR for trisomy
• If this test result is normal, they are tested for chromosome imbalance across the whole genome using array CGH
• Array CGH compares two complete genomes e.g. a sample and a normal control and detects differences in copy number across the entire genome
• Reporting time 10 days
• Imbalances > 400kb and/or within known syndrome regions are reported prenatally

Question 27

When is a rapid FISH test used?

Answer 27

When one partner is a known carrier of a balance chromosome rearrangement

Question 28

What does a rapid FISH test test for?

Answer 28

1) Sporadic primary trisomy of chromosomes 13, 18 and 21 by QF-PCR
2) An unbalanced product of their chromosome rearrangement using rapid interphase FISH - the test is designed specifically for their unique chromosome rearrangement and a result is available in 24h

Question 29

What are non-invasive prenatal testing methods?

Answer 29

• Peripheral maternal blood sampling e.g. biochemical screening - analysis of free fetal DNA in the maternal circulation
• e.g. NIPT and NIPD

Question 30

What non invasive test is used for prenatal diagnosis of monogenic (single gene) disorders?

Answer 30

Non-invasive prenatal diagnosis (NIPD)

Question 31

What does NIPD do?

Answer 31

Identify fetal sequences that are not usually present in maternal circulation through sampling fetal DNA in maternal circulation and sequences maternal plasma (same as NIPT)

Question 32

What can NIPD determine?

Answer 32

1) Can determine fetal Rhesus D status (RHD)

2) Sex of the fetus via detection of the Y chromosome

Question 33

What mutations can NIPD detect?

Answer 33

1) Mutations causing dominant genetic disorders (paternal or de novo) e.g. FGFR2 and FGFR3 related skeletal dysplasia (bespoke testing available)
2) Mutations causing recessive disorders e.g. DMD, spinal muscular atrophy, CAH, CF can be tested using a relative haplotype dosage (RHDO) analysis approach

Question 34

What test is used to confirm diagnosis of aneuploidy?

Answer 34

QF-PCR

Question 35

When is array CGH used?

Answer 35

• Genetic testing for fetuses with ultrasound abnormalities

Question 36

Explain the involvement of clinical genetics in different stages of pregnancy

Answer 36

1) Before pregnancy - pre-conception counselling if they are carriers or know about disease in the family
2) In early pregnancy (first trimester) - abnormalities on prenatal US scan (12 weeks), abnormal result on prenatal diagnosis
3) In later pregnancy - fetal abnormalities

Question 37

Why might a couple/parent involve clinical genetics before conception?

Answer 37

1) Usually FH of genetic disorder
2) Consanguinous couple
3) Known disease(s) prevalent in the community where parents come from e.g. haemoglobinopathies, Tay-Sachs
4) Prospective parent affected with a genetic condition
5) To establish offspring risk
6) Provide counselling
7) Initiate testing if appropriate
8) Facilitate informed choice in future pregnancy

Question 38

Why might a couple/parent involve clinical genetics during pregnancy?

Answer 38

1) Abnormal result on prenatal screening ± prenatal diagnosis and/or US
2) Diagnostic opinion
3) Provide information
4) Facilitate diagnostic investigations
5) Prognostication
6) Treatment options
7) Support

Question 39

When might there be a risk of T21 happening again?

Answer 39

If it is caused by a translocation (5% of cases)