Prenatal screening and diagnosis Flashcards
Discuss identification of structural abnormalities
-Screening in first trimester (2)
-Screening in second trimester (2)
-Rates of structural abnormalities identified for:
-CNS, GIT, Cardiac, NTD, Abdominal wall defects, Limbs and bones
- Screening in the first trimester
-Not done
-Can pick up 50% of major structural abnormalities at NT scan but need further scanning in second trimester to clarify dx and prognosis - Screening in the second trimester
-25% of fetal conditions manifest in the second and third trimesters
-Women with suspected abnormalities should be referred to tertiary centre for confirmation. Detection rate 3 x better - Detection rates for structural abnormalities
-CNS - 75%
-GIT - 50%
-Cardiac - 25%
-Open NTD - 90%
-Abdominal wall defects - 90%
-Limbs - 75%
Discuss genetic carrier screening
-Incidence (1)
-Major types of inheritance in healthy parents (2)
-Who should be screened (3)
-Which conditions are frequently tested for and their carrier incidence (5)
- Incidence
-1:400 people are affected by inherited conditions - Major types of inherited conditions of healthy parents
-X linked recessive
-Autosomal recessive - Who should be screened
-Offer to couples who have do NOT have a an increased risk of being a carrier based on their personal or family histories
-People with increased carrier risk based on personal or family history should be referred to a clinical geneticist for consideration of diagnostic testing
-In NZ screening only funded for haemoglobinopathies. - Which conditions are frequently tested for
-Cystic fibrosis 1:25 NZ, 1:35 Aust - 1:5000 affected
-Spinal muscular atrophy 1:50 - 1:10 000 affected
-Fragile X 1:332 1:7000 affected
-1:240 couples are at risk of having a child with one of these conditions
-1:1000 couples have an affected child
Discuss an approach for genetic carrier screening
-History (2)
-Basic screening (1)
-Additional screening (5)
-Types of additional screening (2)
- History
-Family history of genetic conditions and consanguinity from all couples intending to have a child
-Ideally do prior to conception to increase management options - Basic screening
-Offer all women basic thalassemia screening - free in NZ - Additional screening
-Provide information on carrier screening to all women prior to pregnancy or in first trimester
-Genetic screening should only be offered for conditions that majorly impact quality of life
-Advise that this screening is not funded - Types of additional screening
-Limited panel - CF, Spinal muscular atrophy, Fragile X
-Extended panel investigates
>1000 autosomal and X-linked recessive conditions
Should not be offered routinely as find variants of uncertain significance.
Should only be done with stringent pre and post test counselling
Discuss genetic carrier screening types
-Sequential screening (2)
-Couple screening (4)
- Sequential screening
-Woman is tested first and if carrier then partner is tested
-Lower cost - Couple testing
-One step screening where both ppl are tested at same time
-Faster and less referrals to genetic counselling
-More expensive
-Results invalid if reproductive partners changed
Discuss options if carrier status is detected
- Conceiving naturally and testing child after birth
- Conceiving naturally and dx test in utero - CVS/Amnio
- IVF with pre-implantation genetic testing
- Use donor sperm or egg or embryo
- Adoption
- Not having children
Discuss cystic fibrosis genetic carrier testing
-Gene tested for (1)
-Chance of having a child with CF (1)
-Methods for diagnosis (4)
- Gene tested for
-CFTR - cystic fibrosis transmembrane conductance regulator gene - Chance of having a child with Cf if both parents are carriers - 1:4
- Methods for diagnosis
-Pre-implantation
-Amnio/CVS
-Guthrie heel prick at delivery
-Cannot currently do on NIPT
Discuss genetic carrier screening for haemoglobinopathies
-Type of inheritance
-Type of disease (2)
-Screening method (2)
-Diagnosis
- Inheritance type - autosomal recessive
- Type of disease
-Thalassemias
-Sickle cell - Screening methods
-Haemoglobin electrophoresis
-Cannot do NIPT - Diagnosis
-CVS or amnio
Discuss outcomes of genetic carriers of alpha thalassemia
-Mother (–/aa) and father (–/aa) have two alpha gene. Both missing same two.
-Mother (–/aa) has two alpha genes and father (aa/-a) has 3 alpha genes
-Mother (aa/–) and father (-a/-a) are both missing two alpha genes but not the same ones
- (–/aa) + (–/aa)
-25% non carrier (aa/aa)
-50% carrier (–/aa)
-25% Barts hydrops (–/–) - (–/aa) + (-a/aa)
-25% non carrier
-25% carrier but 3 alpha genes
-25% carrier but 2 alpha genes
-25% (–/-a) - HbH disease - (aa–) + (-a/-a)
-50% carrier with 3 alpha genes
-50% HbH disease
Discuss outcomes for genetic carriers of beta thalassemia
-Carrier with one normal and one abnormal gene for both couples
-Carrier with one normal and one abnormal gene + non-carrier
- Both parents carriers of an abnormal gene
-25% chance - non-carrier
-50% carrier
-25% of beta thalassemia major as both genes altered - One parent a carrier and one 2 x normal genes
-50% non-carriers
-50% carrier of beta thal
Discuss MSS1/MSS2 screening
-Counselling to women (10 points)
- All women should be offered to women in the first trimester
- Should be offered irrespective of clinician’s perception of woman’s likely choice
- Women can opt out of screening - voluntary
- Should include the following in clear simple language
-Description of conditions screened for
-Differences between screening and diagnostic tests
-Pros and cons of different tests
-Possibility that tests can reveal abnormalities other than those expected
-That if the condition is diagnosed then genetic support will be offered
-That management can include TOP and that gestation will impact type of TOP
Discuss MSS1/MSS2
-Disorders screened for
-Factors affecting screening results
-Screening types for multiple pregnancy
- Disorders screened for
-T13, T18, T21 (make up 66% of aneuploidy). T21 makes up 52% - Factors affecting screening
-BMI
-IVF
-Smoking
-Twin and higher order pregnancies - Screening types for multiple pregnancies
-Twins - MSS1/NIPT. Sens reduced to 72-80%
-Triplets - MSS1
Discuss maternal age and aneuploidy
-Used for screening
-Risk of T21 at 20,30, 40, 50yrs
- Used as a screening test
-Part of MSS1 and Mss2
-Used alone has sensitivity of 40% - Risk of T21
-20 years - 1:2000
30 years - 1:900
40 yrs - 1:94
50 yrs - 1:4
Discuss nuchal translucency
-When to perform (2)
-Utility of screening for chromosomal anomalies (3)
-Conditions it is raised in (6)
-Other testing if NT >3.5 (2)
- When to perform
-11-13+6
- If CRL 56-84mm - Utility of screening for chromosomal anomalies
-Part of MSS1 screening
-NT + Maternal age had sensitivity of 75%
-NT >3mm has 10% risk of anomalies, >6mm has a 90% risk - Conditions raised in
-Cardiac dysfunction
-Fetal anemia
-Skeletal dysplasia
-Chromosomal anomalies - Other testing if NT >3.5mm
-Ref to MFM
-Early anatomy - can do on same scan
Discuss MSS1 testing
-What it involves
-When to do it
-Cut off for increased chance
-Sensitivity and specificity and PPV for T21
-Significance of PAPP-A
- What MSS1 involves
-Maternal age + NT + PAPP-A + BHCG
-Can add in additional findings to increase sens - nasal bone, ductus venosus flow, tricuspid valve flow (contraversial) - When to do
-NT 11-13+6 or if CRL >55mm
-Bloods 9-13+6 - Cut off for increased risk - 1:250
- Sensitivity and specificity
-85% sensitive and 95% specific. PPV = 7-10% - Significance of PAPP-A
-Produced by the syncitiotrophoblasts
-Low is abnormal
-<0.3MoM has RR of 2.6 for IUGR
Discuss MSS2
-What it involves
-When to perform
-Cut off for high chance
-Sensitivity and specificity and PPV for T21
- What it involves
-Maternal age + AFP, Inhibin-A, Estriol, HCG - When to perform
-14-20 weeks - Cut off for high chance
-1:250 - Sensitivity and specificity for T21
-Sensitivity 75%
-Specificity - 93%
-PPV 2-3%
What are the soft markers for aneuploidy (5)
- Absent nasal bone
- Echogenic intracadiac focus
- Echogenic bowel
- Ductus venosus wave form
- Tricuspid valve flow
Discuss NIPT
-What the test measures for (1)
-What is fetal fraction (2)
-What influences fetal fraction (5)
-How much fetal fraction is needed for a test (1)
-How is it analysed (1)
- What does it measure
NIPT measures cell free DNA in the maternal plasma from the syncitiotrophoblasts - What is the fetal fraction
Fetal fraction is the amount of placental DNA that makes up all cell free DNA.
At 10 weeks this is about 10% - Factors influencing fetal fraction
-Gestation - fetal fraction increases with advancing gestation
-Maternal BMI (decreased with increasing BMI)
-Low PAPP-A (small placenta and reduced fetal fraction)
-Aneuploidy
-Twins (Less per fetus cfDNA in didi twins) - Fetal fraction required for reporting - 2-4%
- How is it analysed
-Computerised DNA sequencing to look at preselected chromosomes or whole genome
Discuss NIPT
-What does it test for (5)
-How can in be used in clinical practice (3)
-What should it not be used for (5)
- What does it test for
-Trisomy 13, 18, 21
-Fetal sex
-Sex chromosome aneuploidy - but less accurate
-Single gene disorders
-Will be able to look at RhD status - How can it be used
-First and second trimester screening test
-Second tier screening in those who received increased MSS1/2 testing.
-Woman wishing for another level of screening - When should it not be used
-As the basis of TOP
-If a definitive dx is required
-If there is a structural abnormality
-If the MSS1 risk is higher than 1:10
-Not as a substitute for NT scan for twins (need morphology and chorionicity scan)
Discuss NIPT
-Sensitivity
-Specificity
-NPV for low risk result
-PVV for high risk result
-Sensitivity for sex aneuploidy (2)
- Sensitivity - 99%
- Specificity - 99%
- NPV for low risk result - 99.99%
- PPV for high risk result - 81%
- Sensitivity for sex aneuploidy
-89% for XO
-95% for others.
Discuss NIPT
-What are the advantage (6)
-What are the disadvantages (6)
- Advantages
-High detection rate
-Very low false positive rate <1:10,000
-Larger gestational window for screening with earlier detection
-Does not require highly skilled personnel
-Can detect sex chromosome abnormalities
-Potential to expand diagnostic potential - Disadvantages
-Not diagnostic
-Not as accurate for sex chromosome abnormalities - confounded by placental and somatic mosaicism
-1-6% un reportable
-No NT scan so don’t get early anatomy information
-Doesn’t screen for all abnormalities
-Cost
Discuss NIPT
-Amount of tests where a result is not obtained
-What should be done if no NIPT result can be obtained
-Causes of ‘no call’ result
- Amount of no call results - 1-6%
- Management if no call result obtained
-Offer repeat NIPT (50% have result)
-Offer detailed anatomy or diagnostic testing
-Offer MSS - Causes of no call result
-Low fetal fraction at early gestational age
-Sub-optimal sample collection
-Fetal karyotype can result in reduced fetal fraction (T18)
-Maternal obesity
-Multiple pregnancy
Discuss indications for CVS and amniocentesis (6)
-Increased risk of screening - MSS1/2 or NIPT
-Previous chromosomal abnormality
-Parents are affected by genetic condition or are found to be carriers of a recessive trait
-Age >35yrs
-Structural abnormality on USS
-Early onset IUGR
Discuss chorionic villous sampling
-Timing (1)
-Methods (1)
-Advantages (1)
-Disadvantages (5)
-Miscarriage rate (2)
- Timing
-10 - 13+6 - Methods:
-USS guided needle aspiration bx of the placenta. Usually TA but can be TV
-Screen for blood borne viruses prior to procedure - Advantages
-Allows for earlier diagnosis and therefore earlier TOP if desired - Disadvantages
-Subject to placental mosaicism -1-2%
-More technically demanding
-Risk of limb reduction defect if done before 11/40
-Chance of maternal contamination - 1%
-Subject to placental location - posterior might not be possible. - Miscarriage rate
-1-2% but old data - likely <0.5%
-0.22% at specialist centres
-1% if twin pregnancy
Discuss amniocentesis
-Timing (1)
-Method (1)
-Advantages (3)
-Disadvantages (2)
-Miscarriage rates (2)
- Timing
>15 weeks when chorion and amnio have fused - Method
-Sample of amniotic fluid - 20-30mL is drawn under sterile technique with USS guidance
-Screen for blood borne viruses prior to procedure - Advantages
-Lower miscarriage rate
-Not restricted by location of placenta
-Doesn’t pick up mosaicism - Disadvantages
-Late diagnosis
-Increased risk talipes - Miscarriage rate
-1-2% old data Likely <0.5%
-0.11% in specialist centres
-1% if twin pregnancy
-Operator dependant
Discuss CVS and amniocentesis
-Maternal risks (6)
-Fetal risks (5)
- Maternal risks
-Miscarriage
-Chorioamnionitis
-Haemorrhage
-Haematoma
-Rh sensitisation
-Uterine contractions - Fetal risks
-Fetal infection. <1:1000 but higher if blood born infection
-Fetal injury
-Rupture of membranes
-If CVS performed <11 weeks increased risk limb reduction defects
-If amniocentesis performed before 14 weeks - increased fetal loss and talipes
Discuss diagnostic tests used in CVS and Amniocentesis: Conventional karyotype
-What it measures (2)
-How long it takes to process (1)
- What it measures
-Stains chromosomes from cultured fetal cells for microscopic inspection
-Looks at chromosomal number, length, banding pattern to 5-10 mega bases - Processing time
-2 weeks
Discuss diagnostic tests used in CVS and amniocentesis:
Fluorescent in situ hybridisation
-What it looks for (3)
-Benefits (1)
-Limitations (3)
- What it looks for
-Rapid aneuploidy testing
-Can also look for specific microdeletions
-Used in conjunction with karyotype - Benefits
-Quick preliminary diagnosis - 48hrs - Limitations
-Need to interpret false positive results with caution and use in conjunction with karyotype and USS findings
-False negs: only exclude full trisomies so doesn’t rule out structural chromosomal abnormalities
-Results can be inconclusive if mosaicism present
Discuss diagnostic tests for CVS and amniocentesis: Chromosomal microarray analysis
-What it looks at (5)
-What it doesn’t look at (1)
-Limitations (1)
- What it looks at
-High resolution analysis of whole genome
-Looks for copy number variants
-Identifies large (5-10Mbp) and submicroscopic (<5Mbp) DNA variations
-First tier test for fetal structural abnormalities - detects more pathogenic aneuploidies than other methods
-Can look for single nucleotide polymorphisms - What it doesn’t look for
-Can’t pick up balanced translocations - Limitations
-Detection of variants of uncertain significance 5%
Discuss pre-implantation genetic testing
-Indications (4)
-Indication for pre-implantation screening (2)
-Indication for preimplantation diagnosis (1)
-Advantages (1)
-Disadvantages (2)
- Indications
-Increased chance of genetic condition based on personal and family history
-Genetic carrier status
-Multiple miscarriages
-Lack of success with multiple embryo transfer - Indication for pre-implantation screening
-Recurrent miscarriages
-Lack of successful embryo transfer - Indication for preimplantation diagnosis
-Perform in those seeking an embryo transferred which is not affected - Advantages
-Reduces risk of genetic abnormality and can request TOP - Disadvantages
-Higher risk of NIPT failure for NIPT and MSS1 with IVF
-Subject to mosaicism error
Discuss RANZCOG recommendations for prenatal tests for chromosomal abnormalities (17)
- Should be explained and offered to all women in first trimester
- Should be voluntary
- If the test shows increased risk of chromosomal abnormality then genetic counseling should be available
- Acceptable first trimester screens = NIPT or MSS1
- Couples opting for NIPT should be informed about revealing gender, sex anueploidy and possible unknown maternal conditions
- Acceptable second trimester screens = NIPT or MSS2
- The use of NIPT as a second tier screening test for women with increased MSS should be discussed as well as the pro and cons of this vs dx testing vs doing nothing.
- Dx testing with amnio or CVS should be undertaken before definitive management decisions
- Routine screening for genome wide chromosomal abnormalities and microdeletions is not recommended
- For multiple pregnancy first trimester USS should be done regardless of chromosomal screening choices
- NIPT is OK in multiple pregnancy but has increased failure chances
- In triplets and higher screening for chromosomal conditions should be performed with first trimester USS markers
- If direct access to dx testing is preferred this should be allowed with appropriate counseling
- Preconception screening is preferrable to antenatal screening for heritable genetic conditions
- A careful Hx of couples prior to conception of heritable traits should be taken and those with RF should be referred to genetic counseling.
- Carrier screening info for the more common conditions should be available to all women (CF, Fragile X, MSA, Haemaglobinopathies)
- All pregnant women should be offered thalassemia carrier status screening. Specific assays should be considered for high risk ethnic groups
Discuss early pregnancy screening for PET
-Types of screening
-Sensitivity and false positive rates
-What is is predicting
-RANZCOG recommendation
- Types of screening
Clinical risk factors
-Past or FHx of PET, Underlying medical disorder, Multiple pregnancy
-Assessment of HTN, BMI
Combined clinical, USS (Uterine artery doppler) and biomarkers (PAPP-A and PLGF) - Sensitivity and false positive
-May be as high as 95% and 5% respectively. Some issues with the studies - What it is predicting
-Looks at early onset PET <34 weeks. Not predictive of late onset PET (Much more common) - RANZCOG recommendation
-Routine clinical screening for all pregnant women for PET is recommended
-Use of composite screening tools is yet unclear
Discuss fragile X syndrome
-Number who are carriers
-Genetic basis
-Phenotype (5)
-Epidemiology
- Number who are carriers
1:332 (1: 250-800) - Genetic basis
-Trinucleotide repeat disorder (CGG) in FMRI gene. >200 repeats in full mutation
-X linked dominant with reduced penetrance - Phenotype
-Mild to severe intellectual disability
-Most common monogenetic cause of autism
-Long face with prominent jaw and forehead
-Hyperflexible fingers
-Large ears - Epidemiology
1:7000 males 1:11 000 females
Discuss Spinal muscular atrophy
-Carrier numbers
-Genetic cause
-Pathophysiology / phenotype
-Screening considerations
- Carrier numbers
1:50
1:6000 affected - Genetic cause
-Mutations to the SMN1 gene resulting in reduced expression of SMN protein important for nerve conduction to striated muscle
-Some SMA not linked to mutations of this gene - Phenotype
-4 types of SMA defined by timing of onset
-Progressive with weakness and atrophy of spinal and proximal muscles as they do not recognise nerve stimulation
-No impact on intelligence
-Symptoms depend on severity
-Most severe have breathing difficult and can die in infancy - Screening considerations
-Cannot pick up all SMA as may be other causes
-Cannot tell the severity of SMA by screening
