2.4 Ante&Postnatal Screening
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What is Antenatal screening?
Identifies risk of a disorder so that further tests & a prenatal diagnosis can be offered.
What is ultra sound imaging?
Ultrasound scanner is held against woman’s abdomen & picks up high frequency sounds which bounced off foetus and is converted to an ultrasound image
What times are ultrasound scans given during pregnancy?
8-14 weeks to produce dating scan to determine stage of pregnancy 18-20 weeks to produce anomly scan to check for presence of serious physical abnormalities in the foetus
Why are biochemical tests carried out?
To assess risk of chromosomal abnormalities in the foetus taking into consideration the mothers age
Blood & urine tests are used to
Monitor renal, liver & thyroid functions & other biochemical changes
How do false positives/negatives happen?
If a marker chemical was measured at wrong time & found to be high this would be a false positive & if was low this would be a false negative
When should human chronic gonadotropin be high?
Weeks 6-10 if it remains high the baby has Down syndrome
How is time chosen for biochemical tests?
Time is synchronised with ultrasound dating scans
What is a screening test?
To dectect signs & symptoms of conditions
What is a diagnostic test?
Definitive meaning it establishes if a specific condition is present
When are diagnostic tests offered?
If results from screening tests show evidence of a potential problem Family history of a genetic disorder Woman belongs to high risk category eg over 35.
Why do older woman have a higher risk of down syndrome babies?
Germline cells are more prone to having an extra copy of chromosome 21.
Examples of diagnostic tests are
Amniocentesis CVS- Chronic Villus sampling
Amniocentesis is carried out when?
14-16 weeks
What does amniocentesis involve?
Withdrawing a little amniotic fluid containing fetal cells using a syringe to arrange chromosomes into a karyotype to detect chromosomal abnormalities
Amniocentesis increases the risk of what?
Miscarriage
What does CVS involve
Taking a sample of placenta cells using a testube inserted into the reproductive tract. Cells are cultured and uses for karyotyping
When is it carried out?
Can be carried out at 8 weeks pregnant
Downside to CVS
Higher risk of miscarriage ( higher than amniocentesis)
What is a karyotype
A display of a persons chromosome compliment with chromosomes arranged in homologous pairs.
Karyotype of Down syndrome
Extra copy of chromosome 21
Down syndrome is characterised by
Learning difficulties, distinct physical features
Down syndrome occurs in
1 in 800 live births
Karyotype of someone with turners syndrome
Missing an X chromosome