Genetic testing (condensed)

Question 1

What is chorionic villous sampling (CVS)?

Answer 1

CVS is a diagnostic procedure used to obtain foetal tissue for karyotyping, this allows for the examination of chromosomal abnormalities including Down’s syndrome.

Involves an ultrasound-guided biopsy of the placental tissue. This is used when testing is done earlier in pregnancy (before 15 weeks).

Question 2

When in gestational period can CVS be carried out?

Answer 2

Between 11-14 weeks gestation

Question 3

What is the risk of CVS?

Answer 3

Like any invasive procedure, it carries the risk of miscarriage.

Risk of miscarriage is around 0.7% within 14 days and 1.3% within 30 days.

Question 4

What is amniocentesis?

Answer 4

involves ultrasound-guided aspiration of amniotic fluid using a needle and syringe.

This is used later in pregnancy once there is enough amniotic fluid to make it safer to take a sample.

Question 5

At week of gestation is amniocentesis typically carried out?

Answer 5

Beyond week 15

Question 6

What is the foetal anomaly scan?

Answer 6

A prenatal ultrasound performed in pregnancy.

It checks on the physical development of the foetus and can detect certain congenital disorders as well as major anatomical abnormalities.

Question 7

What conditions can be detected on foetal anomaly scan and what is the % probability that these are detected?

Answer 7

Spina bifida – Open spinal defect – 90%

Anencephaly – Defect in the formation of the skull – 99%

Hydrocephalus – Excess fluid within the brain – 60%

Congenital heart disease – Major heart defect – 60%

Down’s syndrome – May be associated with heart, limb, and bowel problems – 50%

Cerebral palsy – Severe learning difficulties – not seen

Autism – not seen – Fetal echocardiography

Question 8

When is foetal anomaly screening scan usually carried out?

Answer 8

Between weeks 18-21 gestation (usually 20 weeks)

Question 9

What is the difference between a screening and diagnostic scan?

Answer 9

Both screening and diagnostic tests are performed during the pre-natal period.

Screening tests are not diagnostic. They can provide information regarding the risk of a baby having a certain disorder or condition.

Only diagnostic tests are definitive and can identify if a baby does have a birth defect.

Question 10

What is the first line and most accurate screening test used for the foetus?

Answer 10

Combined test

Question 11

When is combined test typically carried out

Answer 11

Between 11-14 weeks gestation

Question 12

Combined test involves combining results from ultrasound and maternal blood tests. True/false?

Answer 12

True

Question 13

For the combined test what does ultrasound measure?

Answer 13

Ultrasound measures nuchal translucency, which is the thickness of the back of the neck of the fetus.

Down’s syndrome is one cause of a nuchal thickness greater than 6mm.

Question 14

For the combined test what does the maternal blood tests measure?

Answer 14

Beta‑human chorionic gonadotrophin (beta-HCG) – a higher result indicates a greater risk

Pregnancy‑associated plasma protein‑A (PAPPA) – a lower result indicates a greater risk

Question 15

When is combined test usually performed?

Answer 15

Between 11 and 14 weeks gestation

Question 16

When is the triple test usually carried out?

Answer 16

Between 14 and 20 weeks gestation

Question 17

Triple test only involves carrying out maternal blood tests, what are the 3 blood tests?

Answer 17

Beta-HCG – a higher result indicates greater risk

Alpha-fetoprotein (AFP) – a lower result indicates a greater risk

Serum oestriol (female sex hormone) – a lower result indicates a greater risk

Question 18

When is the quadruple test usually performed?

Answer 18

Between 14-20 weeks gestation (same as triple test)

Question 19

Quadruple test is very similar to the triple test, what is the one difference?

Answer 19

Involves 4 maternal blood tests (hence the name)

Also includes maternal blood testing for inhibin-A.
A higher inhibin-A indicates a greater risk.

Question 20

Why are screening tests done before diagnostic tests?

Answer 20

Screening tests provide a risk score for the fetus having Down’s syndrome.

When the risk of Down’s is greater than 1 in 150 (occurs in around 5% of tested women), the woman is offered amniocentesis or chorionic villus sampling (dependant on gestational age).