Prenatal Screening & Diagnosis Flashcards
What is the purpose of screening tests?
- screening tests are used to look for an abnormality / certain condition
- they are NOT definitive and another test is required to confirm the diagnosis
- they are used to dictate the clinical pathway and determine whether further testing is required
What do screening tests rely on?
- they rely on factors produced by the foetus / placenta that diffuse across the placenta and enter the maternal circulation
- increased or decreased levels of these factors may indicate an abnormality
- increased / decreased levels of the same factor can be indicative of different conditions, so the change from normal is measured
What is meant by diagnostic testing?
- diagnostic tests are used to confirm a suspicion of an abnormality / condition
- they have a high degree of specificity but there are also associated risks
What are the 2 main reasons why screening tests are important?
- to determine the health / condition of the unborn foetus
- to avoid / manage potentially untoward outcomes for the foetus, mother or both
- it allows for parents to make informed choices about their pregnancy, depending on whether it is deemed to be high-risk or not
What is the NICE recommendation for prenatal screening in England?
- all pregnant women are offered Down’s syndrome screening in the first trimester
- this can be performed via 2 different methods, depending on when the woman presents:
- the combined test
- the quadruple serum test
What is involved in the combined test for Down’s syndrome?
What is the detection rate?
- this involves a nuchal translucency measurement and maternal serum markers at 12 weeks
- the serum markers measured are:
- human chorionic gonadotrophin (hCG)
- pregnancy-associated plasma protein A (PAPP-A)
- it has an 80% detection rate and gives an early indication, but is also associated with 5% false positive rate
What is involved in the quadruple serum test and when can this be performed?
- this involves a blood test to measure 4 serum factors:
- human chorionic gonadotropin (hCG)
- alpha-fetoprotein (AFP)
- inhibin-A
- estriol (E3)
- it can be performed up to 24 weeks, but is most accurate between 15-18 weeks
When is the quadruple serum test used?
What is its drawback over the combined test?
- the quadruple test is less accurate than the combined test
- it can be used for women who present later
- if the results of the combined test are positive, the woman will still be sent for a quadruple serum test at 15-18 weeks
What are both the quadruple serum and combined test followed up by?
- both tests are followed up by a foetal anomaly USS at approx 20 weeks (18-21)
During which period can the combined test be performed?
What 2 conditions can the results raise a suspicion of?
- usually performed at 12 weeks, but can be performed between 10-14 weeks
- can be used to raise suspicion of Down’s syndrome (trisomy 21) and Edward’s syndrome (trisomy 18)
- PAPP-A is decreased from the norm in both trisomy 21 and 18
- hCG is elevated in trisomy 21
- hCG is depressed in trisomy 18
What factors are measured in the quadruple serum test and what conditions does this screen for?
- performed in 2nd trimester between 15-22 weeks to look for levels of:
- hCG
- estriol (E3)
- alpha-fetoprotein (AFP)
- + / - Inhibin A
- elevated levels of AFP indicate neural tube defects (NTDs)
- AFP, E3 and hCG are all depressed in trisomy 18
- AFP and E3 are depressed in trisomy 21, but hCG and inhibin A are elevated
Why is accurate dating of pregnancy vital for interpreting screening tests?
- accurate dating of pregnancy is vital as levels of serum markers are dependent on foetal age
- there are peaks and troughs of serum markers throughout pregnancy, so values need to be compared to what is expected at that point in pregnancy
What are the 3 major benefits of performing USS?
- safe as it is non-ionising
- cheap to perform
- image quality and detail have improved massively recently
- X-ray and CT scans may be more helpful in diagnosis, but they are radiating and carry a risk of harm to the unborn foetus
How is gestational age estimated using USS?
Why is this clinically important?
- the most accurate way to date a pregnancy is by measuring the crown-rump length (CRL)
- bi-parietal diameter (BPD) and femoral length (FL) are less accurate when used alone
- usually CRL, BPD and FL are combined with other clinical factors to estimate gestational age
- this is important in dating a pregnancy** and **interpreting serum markers
How is nuchal translucency measured?
When is it measured and why is it significant?
- this measures the thickness of fluid present in the subcutaneous tissue in the nuchal region
- it is performed between 11-14 weeks, but usually at 12 weeks alongside the combined test
- nuchal translucency > 3mm** is strongly associated with **cardiovascular system defects and chromosomal abnormalities
- it is particularly indicative of Down’s syndrome
What are the 2 types of diagnostic tests and why are they performed?
What must the mother be aware of before agreeing to these tests?
- chorionic villus sampling (CVS)
- amniocentesis
- they are performed to confirm any suspected abnormality
- diagnostic tests are INVASIVE procedures and carry a risk of damage to the foetus and foetal death
When are diagnostic tests routinely offered?
- diagnostic tests are routinely offered regardless of screening tests when:
- mother is >35 years of age
- previous child with a congenital abnormality
- one of the parents has a chromosomal disorder
- they are also offered when a screening test is suggestive of a congenital abnormality being present
What is involved in chorionic villus sampling?
When is this performed?
- a needle is inserted transabdominally or transvaginally to obtain a small sample of villus tissue from the placenta
- the placental cells are cultured and karyotyped for chromosomal, genetic and molecular analysis
- CVS is performed at 10-12 weeks
What are the 3 major drawbacks of using CVS as a diagnostic test over amniocentesis?
- it is less accurate than amniocentesis and may have to be repeated
this is due to placental mosaicism or contamination with maternal cells
- there is a higher risk of pregnancy loss (1-2%)
- the risk is higher when the transcervical method is used
- it cannot be used to sample AFP
How is amniocentesis performed?
When is it performed and why can it not be performed earlier on?
- a needle is inserted through the abdominal wall, uterine wall and membranes to take a sample of amniotic fluid
- it is performed at 15-16 weeks
- it cannot be performed prior to this as insufficient amounts of amniotic fluid have developed
- removal of amniotic fluid prior to 15 weeks could be detrimental to the health of the foetus
What are the risks associated with amniocentesis?
- risk of Rhesus sensitisation if there is mixing of foetal and maternal blood
- Rh prophylaxis is given where appropriate
- small risk of pregnancy loss (0.06%)
- risk of injury to foetal organs if USS guidance is not used during the procedure
How is karyotyping performed?
- cells are cultured for 2-3 days
- colchicine is added, which prevents spindle formation
the cells are arrested in metaphase, allowing genetic material to be karyotyped and sequenced
- hypotonic solution is added to rupture the nuclear membrane
- the genetic material is withdrawn and placed on a slide and stained with Giesma
- a light microscope is used to take a photomicrograph, allowing the chromosomes to be visualised
What are the 2 different types of chromatin that are present within a chromosome?
Heterochromatin:
- represented by the dark bands on the chromosomes
- it is highly condensed, contains A-T rich bonds between nucleotide bases and few genes
Euchromatin:
- represented by the light bands on the chromosomes
- it is less condensed, contains G-C rich bonds between nucleotide bases and many genes
How is fluoresence in situ hybridisation (FISH) performed?
- this method uses small sections of cloned DNA called probes
- probes are labelled with a fluorescent marker or antibody
- the probe will then anneal (bind) to a matching region of DNA (hybridisation)
How is chromosome painting used during FISH and what is the benefit of this?
- several probes are used for each chromosome, allowing for quick and easy detection of structural abnormalities
- during the cloning DNA process, it is easy to see where fluorescent tags may be missing
How are non-invasive screening and diagnostic tests performed?
- short strands of foetal DNA were discovered in maternal blood plasma
these foetal cells crossing the placental membrane are sparse, so it is difficult to obtain sufficient amounts of genetic material for analysis
- cell-free DNA strands also cross the placenta and are far more abundant
these come from foetal trophoblast cells that have passed into the maternal circulation and undergone apoptosis
- cell-free DNA is abundant so is easy to sample and can be detected as early as 4-5 weeks
What conditions are screened for through the foetal anomaly screening at 20 weeks?
- anencephaly
- myelomeningocele
- gastroschisis
- cleft lip
- bilateral renal agenesis
- diaphragmatic hernia
- serious cardiac abnormalities
