Antenatal Screening

Question 1

What is the purpose of screening?

Answer 1

to identify individuals at a higher risk of a disease or condition

Question 2

What is the purpose of screening for chromosomal abnormalities during pregnancy?

Answer 2

to establish which women should receive more invasive tests to establish a definitive diagnosis

Question 3

What is involved in the combined test?

When is it performed?

Answer 3

• this is the most accurate screening test for Down’s syndrome
• it is offered to all women between 11 and 14 weeks gestation
• it involves USS and maternal blood tests

this is an OPT IN test

Question 4

What is the role of USS in the combined test?

Answer 4

• it measures the nuchal translucency
• this is the thickness of the back of the neck of the foetus
• Down’s syndrome causes nuchal thickness > 6mm

Question 5

What maternal blood tests are included in the combined test?

Answer 5

beta-HCG:

• higher result = greater risk

PAPP-A:

• lower result = greater risk

beta-HCG = beta-human chorionic gonadotrophin

PAPP-A = pregnancy-associated plasma protein-A

Question 6

What is done to screen for Down’s syndrome if the woman misses combined screening?

Answer 6

• the triple or quadruple test can be performed between 14 to 20 weeks gestation
• these involve blood tests ONLY
• they are not as accurate as combined screening

Question 7

What blood tests form part of the triple test?

Answer 7

beta-HCG:

• higher result = greater risk

serum oestriol:

• lower result = greater risk

alpha-fetoprotein (AFP):

• lower result = greater risk

Question 8

How is the quadruple test different to the triple test?

Answer 8

• it is the same, but has an additional blood test
• it also includes inhibin-A
• higher result = greater risk

Question 9

When is someone deemed high risk of Down’s syndrome after screening?

What additional test are they offered?

Answer 9

• the screening tests provide a risk score for the foetus having the condition
• if the risk is > 1 in 150, they are high risk
• high risk women are offered amniocentesis or chronionic villus sampling (CVS)

Question 10

How can CVS / amniocentesis give a definitive answer about Down’s syndrome?

Answer 10

• they involve taking a sample of fetal cells
• karyotyping can then be performed
• this provides a definitive diagnosis

Question 11

What is chorionic villus sampling (CVS) and when is it performed?

Answer 11

• it involves an ultrasound-guided biopsy of placental tissue
• performed when testing is earlier in the pregancy (before 15 weeks)

Question 12

What is amniocentesis and when is is performed?

Answer 12

• it involves ultrasound-guided aspiration of amniotic fluid using a needle & syringe
• it is performed later in pregnancy (after 15 weeks) than CVS
• there needs to be enough amniotic fluid present in order for it to be safe to sample

Question 13

What is a new alternative that is being introduced in place of invasive testing for high risk mothers?

Answer 13

non-invasive prenatal testing (NIPT)

• this is NOT diagnostic, but gives a very good indication as to if the fetus is affected

Question 14

What is involved in NIPT?

Answer 14

• a blood test is taken from the mother
• this contains fragments of DNA
• some DNA fragments come from placental tissue and represent fetal DNA
• these can be analysed