7. Prenatal Screening Flashcards
(28 cards)
How many babies are born/year
70,000
What percentage of babies have birth defects
4%
What kind of birth defects exist?
Structural Functional Genetic Chromosomal Biochemical
What is the most common chromosomal abnormality?
Trisomy 21 (1 in 600 ~ 1 in 1000) 2nd most common XXY (Klinefelter syndrome)
What information is important from taking a family history?
- Should ask a 3 generation family history Including age of diagnosis
What questions should you ask when taking family history?
- Inherited conditions - Down syndrome and other chromosomal conditions - Other birth defects, e.g. spina bifida, cleft/lip palate, cardiac defects, intellectual disability - Recurrent miscarriage - Unexplained perinatal deaths - Consanguinity - Ethnic background - IVF/Donor gametes
What is the risk of giving birth to a child with Down Syndrome?
Increases exponentially from 1 in 1411 at age 20-24 to 1 in 32 at age 45
What is the prevalence of neural tube defect?
1 in 500 o Spina bifida o Anencephaly o Encephalocele - Folate Should be taken before and during pregnancy
What are the benefits of prenatal testing?
- Offers reproductive choice - Risk information - Choice of termination - Prepare psychologically - Help plan delivery and management - Provide reassurance
What are the 2 major types of prenatal testing?
Screening test - Offered to all pregnant women - Non-invasive - Taken by up to 80% pregnant women Diagnostic tests - Offered to women who are at increased risk of having a child with a birth defect - Invasive
What is the 1st trimester combined screening?
- Blood taken at 10 weeks o 2 biochemical analytes measured - Ultrasound performed at 11 -13 weeks. o Nuchal translucency measured (oedema that occurs behind fetal neck) o Crown rump length (dating) - Other factos including maternal age Results/Cutoff - T21: 1 in 300 - T18: 1 in 175 Detection rate for T21 is 93% - 4.6% false positive result
What is the 2nd trimester combined screening?
- Blood taken at 14-20 weeks (15-17 ideal) o 4 biochemical analytes measured - Maternal age Cutoff - T21 1 in 250 - T18 1 in 200 - Neural tube defects >2.0MoM Detection rate for T21 is 85%, with 6.5 false positive rate Detection rate for neural tube is 93% but ultrasound is recommended
What is NIPT/S?
(non-invasive prenatal testing/screening) – using Cell-free fetal DNA/RNA in maternal blood - Non invasive - ~10 weeks gestation - Uses new sequencing technology
What does Chronic cillus sampling (CVS) involve?
- From 11 weeks gestation - Placental tissue - Ultrasound - Invasive 1% miscarriage - If termination of pregnancy requested, dilation & curettage under general anaesthetic (

What is Amniocentesis?

15-16 weeks gestation
Amniotic fluid (20ml) containing sloughed off fetal cells removed
Ultrasound
0.5% miscarriage
If TOP requested, by prostaglandin (>16 wks)
What is fish?
- Fluorescence in site hybridization
- Using probes on interphase cells to look for abnormalities in chromosome numbers
What recessive conditions often present without a Fx?
- Haemoglobinopathies (1 in 20 – ethnicity)
- Ashkenazi jewish panel (7 conditions, 1 in 25 to 1 in 100)
- CF (1 in 25 – Northern Europe)
- Spinal muscular atrophy (1 in 40)
- Fragile X syndrome (1 in 200)
What is Robertsonian translocation?
Rearrangement on Acrosomic Chromosomes
What re acrocentric chsomosomes?
Chromosomes with a small P arm
Examples (13, 14, 15, 21, 22)

Balanced vs unbalanced translocation in Trisomy 21
- Most common is 14;21
- If they are balanced, has not loss any genetic material it will not cause any problems for the individual but may cause problems for offspring when inherited
- Balanced translocation = carrier status and that will have no implication to individual unless genetic information has been lost
But it can occur in the offspring - Unbalanced translocation between chromosome 14 and 21 leads to trisomy 21 = Down Syndrome

How does unbalanced translocation cause Down Syndrome?

Microarrays (DNA chips)
- DNA sequences and/or gene expression
- Can look at SNPs
- Can look at larger changes – number of chromosomes and copy number variations
What are chromosomal microarrays?
- Array CGH (comparative genome hybridization) and SNP arrays
o Also called molecular karyotyping - Probes (25 base oligonucleotides along entire genome)
- Variable resolution
- For deletions and microdeletions
When should prenatal molecular karyotyping be performed?
- a fetal abmormality identified on ultrasound scan
- Nuchal translucency is >3.5mm
- A banded (classical cytogenic) karyotype identifies a complex change
- A family member has a microdeleton syndrome and a pregnancy is at risk