Antenatal Screening Flashcards
What is the antenatal care schedule (8-10 routine appointments)?
1) Booking appointment
2) 16 weeks
3) 18-20 weeks
4) 28 weeks
5) (31 weeks if nulliparous +) 34 weeks
6) 34 weeks
7) 36 weeks
8) 38 weeks
9) (40 weeks if nulliparous +) 41 weeks if not gone into labour
What are the two scans to screen for fetal anomalies?
1) 12 weeks (11-13) - blood test and early pregnancy US
2) 18-20 weeks - detailed US for fetal anomalies and screening, internal structures checked in a lot of detail
What haematological conditions are screened for in pregnancy?
1) Anaemia
2) Haemoglobinopathies e.g. sickle cell anaemia, thalassaemia esp. in at risk individuals
What infectious diseases are screened for in pregnancy?
1) Syphilis
2) Hep B
3) HIV
4) Rubella susceptibility
When should you offer gestational diabetes screening to a woman?
1) BMI > 30
2) Previous macrosomic baby weighing ≥ 4.5kg
3) Previous gestational diabetes
4) FH of diabetes (first degree relative)
5) Family origin with high prevalence of diabetes e.g. South Asian, Black Caribbean, Middle Eastern
Screening for which genetic conditions is offered to all women?
1) Down’s syndrome (T21)
2) Edward’s syndrome (T18)
3) Patau syndrome (T13)
When is trisomy screening done?
- The combined test in the first trimester from 10-14 weeks
- Can also be performed in the second trimester
Describe the features of the trisomy screening tests
- Can choose to have screening for all conditions, T21 only, T18 and T13 only or no screening
- One risk result generated for T21 and another one for T18+T13
- It assess the risk of the fetus having these conditions and cannot establish the presence or absence of these conditions
- i.e. higher risk result does not mean fetus has condition and lower risk result does not exclude the possibility
- Informed consent is importnat
What are the aspects of the risk determinants in the combined test for the trisomies?
1) Maternal age
2) Measurement of nuchal translucency (12 week scan)
3) Gestational age from the length of the fetus (CRL length)
4) The level of PAPP-A and free beta-hCG (hormones) in maternal blood
How good is the detection rate of T21 in the combined test?
85-90% detection rate with ~3% false positive
What do maternal PAPP-A and free beta-hCG levels and nuchal translucency as part of the combined test indicate?
1) PAPP-A is decreased in all 3 trisomies esp. T13 and T18
2) Free beta-hCG is increased in T21 and decreased in T13 and T18 (more decreased in T18)
3) Nuchal translucency > 4mm - increased in all trisomies esp. T21 and T18 (also in Turner syndrome)
Explain how results are given and used in the combined test?
- The result is given as a risk score e.g. 1:20 for T21 and 1:300 for T18/13
- The cut-off for a screen positive/high risk result is ≥ 1:150
- The cut-off for a screen negative/low risk result is < 1:150
- Patients given a positive screening result are offered either an invasive test (CVS/amniocentesis) or non-invasive prenatal testing (NIPT) privately
Which trisomy does the quadruple test screen for?
T21
Describe the quadruple test used for screening
- 14-22 weeks
- Measures AFP, hCG, unconjugated estriol (uE3) and inhibin A
- T21 detection rate = 80% (FP rate = 3%)
- Can also detect spina bifida
What do AFP and hCG stand for?
AFP = alpha fetoprotein hCG = human chorionic gonadotropin
What would you see on the 20 week anomaly scan in trisomy fetuses?
- ~ 50% of fetuses with T21 will have features on US
- All fetuses with T18 or T13 would be expected to have abnormal features detected on US
What % of fetuses with Down’s syndrome will not be detected by screening?
10-20%
Who is offered screening for chromosomal abnormalities?
All pregnant women
What is nuchal translucency?
- Fluid level at the back of the neck
- Normal range is 1-2mm (>2.5mm is abnormal)
- Down’s syndrome will be > 4mm (bigger area of black on US)
- Risk of adverse outcomes (abnormality or death) increases with NT thickness e.g. ≥ 5.5mm = 80% risk
How are most T13 and T18 babies picked up?
On scan as they have structural abnormalities
What is the baseline risk of T21 in a 20 year old vs 40 year old?
1:1000 vs 1:100
Who gets screened for sickle cell and thalassaemia?
- High incidence = all pregnant women
- Low incidence = based on ethnic origin
When does screening for sickle cell and thalassaemia happen?
Pre-conception to 10 weeks
How is screening performed for sickle cell and thalassaemia?
Blood test (haemoglobinopathic)
What do you do if the patient screens positive for sickle cell and thalassaemia?
1) Offer partner testing - can see if dad is also a carrier like mum
2) Offer prenatal diagnosis - CVS or amniocentesis
3) If fetus is affected - termination or early specialist care
Who gets screened (diagnostic) for infectious diseases?
Offered to all women
What infectious diseases are screened for and what is the purpose of screening?
- Syphilis, Hep B, HIV
- To prevent transfer to the fetus
When does screening for infectious diseases happen?
At booking
How is screening for infectious diseases performed?
Serology
What happens if the woman tests positive for HIV?
- Maternal treatment
- High risk antenatal care
- Prevent transmission to neonate
What happens if the woman tests positive for Hep B?
- Changes antenatal care to prevent transmission
- Neonate receives vaccine
What happens if the woman tests positive for syphilis?
Maternal treatment to prevent congenital syphilis
What other condition is screened/risk calculated for in pregnancy?
Pre-eclampsia
What are high risk factors for pre-eclampsia?
1) Hypertensive disease in a previous pregnancy
2) CKD
3) Chronic hypertension
4) Diabetes
5) Autoimmune disease e.g. SLE, APS
What are moderate risk factors for pre-eclampsia?
1) First pregnancy
2) Age > 40 (every 10 years above 30 years)
3) BMI > 35 (every 10kg above 70kg)
4) Inter-pregnancy interval > 10 years
5) FH of pre-eclampsia
6) Conception by IVF
7) Racial origin - Afro-Caribbean, South Asian
Which patients are high risk for pre-eclampsia in need of aspirin at < 16 weeks (reason for screening)?
1) Pre-eclampsia in > 2 previous pregnancies
2) Pre-eclampsia at < 34 weeks in previous in previous pregnancy
- Aspirin can help if given early enough (before 16 weeks)
What is AFP?
A protein made in the liver of a fetus which reaches maternal serum directly across the placenta from amniotic fluid, indirectly from amniotic fluid across the chorion and amnion and uptake by maternal vasculature in uterine decidua
How do levels of AFP change in gestation?
Lower in 2nd than 3rd trimester
How is AFP used to screen for congenital abnormalities
- Used to screen for neural tube defects (NTDs) and T21
- There is a correlation between maternal serum AFP levels and severity of neural tube defect
- The higher the AFP at 16 weeks the more severe the NTD i.e. at higher AFP levels anencephaly occurs rather than spina bifida
- Accurate estimate of gestation at which the sample is obtained is essential for interpretation of the serum marker levels
Describe the features of a genetic assessment used to screen for genetic conditions and discussion with the family
1) Detailed FH
2) Diagnostic considerations and investigations
3) Risk assessment
4) Counselling
5) Genetic testing (if available and appropriate)
6) Post-test counselling
7) Family screening if appropriate
What is NIPT and what does it screen for?
- Non-invasive prenatal testing (screening)
- Cell-free DNA testing
- It is aneuploidy screening for T21, 18 and 13 (+ X/Y)
- Screening test - FPs and FNs possible
- Not routinely offered on NHS, some centres or private
- Will be offered to high risk women > 1:150 on serum screening on NHS
How does NIPT work?
- Small fragments of cell-free DNA (cfDNA) from the placenta enter and are present in the maternal circulation
- 5-20% of the cfDNA is fetal (placental in origin), 80-95% is maternal
- Sequence maternal plasma
- The free fetal DNA in the maternal blood can be isolated and amplified and tested for chromosomal aneuploidies (or extra/missing fetal DNA segments)
- Calculate the proportion of T21 sequences and compare to reference samples
- A T21 pregnancy exhibits a higher proportion of chromosome 21 fragments in the maternal circulation
When can NIPT be done?
10 weeks onwards (10-11 weeks)
What are the detection rates of NIPT for the different trisomies?
- > 99% detection rate for T21
- ~93% detection rate for T18
- ~80% detection rate for T13
What is done when high risk results from NIPT are found?
Confirmation fo high risk results by invasive diagnostic testing (CVS or amniocentesis)
When is NIPT not recommend?
For US abnormal pregnancies - just invasive testing is recommended
For who is NIPT mainly used?
Couples with anxiety
What kind of heart defect may suggest T21 at the 20 week scan and lead to amniocentesis diagnosis?
Atrial septal defect
Who is offered screening for fetal anomalies?
All pregnant women
When does screening for fetal anomalies happen?
- Between 18 and 22 weeks
- Earlier detection is becoming more common e.g. at 12 weeks if have more time
How is screening for fetal anomalies performed?
US - structured review of organ systems
Why do detection rates of fetal anomalies vary?
1) Vary by exact disease and gestation
2) Skill and training of sonographer
3) Quality of US machine
4) Maternal BMI
What is the benefit of NIPT?
Avoids an invasive test, trisomies can be detected v early on
