Prenatal Testing And Screening

Prenatal Testing And Screening
By:Adel D. Gilbert, MS, CGC
Genetic Medicine Education Coordinator
Institute of Genetic Medicine
Johns Hopkins University

Genetics Board Review Lecture
Lecture Outline
* Definitions
* Age related risks
* Etiology and phenotype of chromosome anomalies
* Risks, phenotype and testing options of ONTD
* 1st and 2nd Trimester Prenatal Testing Options
* 1st and 2nd Trimester MS Screening Options
* New approaches to combining these tools
* Ultrasound as a screening tool

Testing Defined
Screening Defined
* Identify those at increased risk who are not be perceived to be at risk
* Does not dx definitively
* Follow-up options available for definitive information
* Sensitivity=True positives/all affected
* Specificity=True negatives/all unaffected
Baseline Risk for Having a Child With a Serious Birth Defect
Phenotype
* Moderate mental retardation
* Characteristic facies
o upslanting palpebral fissures
o epicanthic folds,
o midface hypoplasia,
o macroglossia
* Congenital malformations
o heart (30-40%), atrioventricular canal
o gastrointestinal tract, such as duodenal stenosis or atresia, imperforate anus, and Hirschsprung disease
o Leukemia (both ALL and AML) 10-20x
o acute megakaryocytic leukemia occurs 200 to 400 times more frequently in the Down syndrome
o 90% have significant hearing loss, usually of the conductive type
Facial
* microcephaly with prominent occiput
* narrow bifrontal diameter
* short palpabral fissures
* low-set malformed ears
* cleft lip +/- palate
* narrow palatal arch
* micrognathia

Skeletal
* neck
* webbed
* chest
* short sternum
* widely spaced nipples
* hips:
* small pelvis, congenital dislocation of the hips, limited hip abduction
* extremities:
* phocomelia
* rockerbottom feet or equinovarus short dorsiflexed big toes fixed flexion deformity of the fingers (overlapping of the 2nd and 5th fingers over the 3rd and 4th fingers)simple arch pattern of the fingers and toes

hypoplasia of fingernails single crease of 5th finger or all fingers (absence of interphalangeal flexion creases)
Trisomy 18
Kleinfelter syndrome
Miscarriage
Turner syndrome
Neural Tube Defects
* Second most common major congenital defect (1-2/1000)
* Not a chromosome anomaly
* Routinely tested and screened for in pregnancy
* Failure neural tube to close at 28 days gestation
* 20% are closed lesions and difficult to detect prenatally

Open Neural Tube Defects
Closed lesions
Open lesions
INDICATIONS FOR PRENATAL DIAGNOSIS
PRENATAL DIAGNOSTIC PROCEDURES
* AMNIOCENTESIS
* CHORIONIC VILLUS SAMPLING
* PERCUTANEOUS UMBILICAL CORD SAMPLING

AMNIOCENTESIS
ULTRASOUND GUIDED
AMNIOCENTESIS
Amniocentesis Testing
* Chromosome analysis
* AF-AFP levels
* Acetylcholinesterase
* Risk of miscarriage associated with procedure 1/100-1/400
Advantages
* Highly reliable results 99+%
* Familiar
* Long standing reputation
* NTD detection
Disadvantages
* Late in gestation
o Decision making
o Privacy
o Mom feels movement
* Fear of needles
* Needle invades the sac

Fetus: 12 weeks gestation
Transcervical
Chorionic Villus Sampling
Transabdominal
Performed >10 wks-13 weeks
Chromosome analysis
Risk 1/100-1/200
* trophoblastic shell cells
* Syncitiotrophoblasts – poly-proliferate tissue type=directs
* cytotrophoblasts
* Mesodermal core=tissue culture
* frorm finger-like extensions

Disadvantages
* Placental mosaicism 1% of CVS is confirmed in the fetus ~ 10-40%
* Second trimester amniocentesis mosaicism ~ 0.1-0.3% & confirmed in a fetus up to 70% of the time.
* ?LRD risk prior to 70 days gestation (10 weeks)
* Higher loss rate
* Less access to procedure
* Higher chance of insufficient sample
* Early test=risk of sampling a fetus potentially destined to miscarry
* No ONTD testing
* More risk of vaginal bleeding
* Speculum

Benefits
* Earlier in gestation
* rapidly growing cell cultures practically free of maternal cell contamination
* an efficient direct method to obtain high quality metaphases from the of the syncitiotrophoblasts tissue which the fetal karyotype is defined within a few hours of chorionic villi sampling (specialty cyto techinque)
* is suitable for a rapid, direct diagnosis of the related metabolic diseases.
* placental mosaicism (trisomic rescue in fetus) can increase the risks of genetic abnormalities such as uniparental disomy

Fetal Blood Sampling
Percutaneous Umbilical Cord Sampling
(PUBS) or Cordocentesis
* ~2% risk of loss
* Technically difficult prior to 20 weeks
* Blood disorders such as hemophilia and anemia
o Useful for detection of Rh isoimmunization of the fetus (blood cell count and oxygen level)→erythroblastosis fetalis (HDN)
* Chromosomal abnormalities Fetal karyotype in 48 hours
* Infections such as toxoplasmosis and rubella.
* The procedure is also used to perform blood transfusions to the fetus and to administer medication directly into the fetal blood supply.

Reproductive Decision Making
RISK Fetal Aneuploidy
Procedure Related RISK
TO TEST OR NOT TO TEST
* I want to know
* The benefits outweigh the risks
* Options are desirable
* Because my doctor says so…..
* Not sure I want to know
* Risks are a big worry
* Options stink
* Because my doctor says so….

SECOND TRIMESTER
MATERNAL ANALYTES FOR ANEUPLOID
SCREENING
FETAL
Alpha-fetoprotein- AFP
Estriol- uE3
PLACENTAL
Estriol- uE3
Human chorionic gonadotrophin- hCG
Inhibin-A

2nd Trimester MSS Overview
Used for detection of:
+ ONTD
+ Down syndrome
+ trisomy 18
+ Smith-Lemli-Opitz syndrome
Serum Marker
Smith Lemli Opitz Syndrome
* Defect enzyme in the conversion of 7-dehydrocholesterol to cholesterol.
* Affects 1 in 20,000 to 40,000 births
* Autosomal Recessive
* Mental Retardation
* Slow growth
* Heart defects
* Facial cleft
* Screen positive women have uE3 < 0.4 MoM
* ~2% baby affected
* Testing AF for 7-8- dehydrocholesterol (7/8-DHC) levels

Turner T13 Triploidy Pregnancy complications
ONTD screening
Detection Rates
MSAFP
Add Ultrasound
Screening for DS
2nd Trimester 1/270 Cut-off
First Trimester Integrated Screening For Trisomies =FIRST
Nuchal Translucency (NT)
First Trimester Screening
Down syndrome DR ~1:270 Cut-off
Nasal bone
Recommendations 1st Trimester Nuchal >3.5
* CVS
* Targeted ultrasound evaluation 18-20
* Echocardiogram
* Residual 5-6% risk neonate may have a yet undefined genetic syndrome…
Fetal Nasal Bone
* 65% DS have absent nasal bone
* General population 1%
o African Americans 8%-10%
* Secondary screen
* Difficult to obtain
* Higher false positive in 1st

Integrated Screening
PAPP-A and Nuchal
Quad Screen
Screen Positive
Screen Negative
Timeline weeks
Decision Making
Advantages
* Increases detection rate
* Decreases false positive rate (fewer tests and fewer procedure related losses)
Disadvantages
* Long wait time for result
* Unable to utilize CVS and early detection
* Late GA by the time amnio results final
Contingent

First Trimester Screening
High risk
Low Risk
Intermediate
Triple Screening Integration
Offer CVS
High risk
Low Risk
US and Amnio
Stop
Advantages
* Increase detection rate 90%
* Decrease FPR 2%
* Reduce the number of amnios performed in low risk pregnancies
Disadvantages
* New (limited data)
* Hard to determine uptake
* Offered at few institutions
RISK OF ANEUPLOIDY BASED ON GA AND ANOMALY

3D Ultrasound
Fetal Face
24 weeks Gestation
Fetal MRI
FETOSCOPY
Amnion (stuck twin)
Umbilical cord

Prenatal Testing And Screening.ppt