Lecture 28: Prenatal Tests

Question 1

What is the chance of a baby being born with an abnormality?

Answer 1

1/30

Question 2

What is the chance of miscarriage?

Answer 2

1/4

Question 3

What is the chance of serious intellectual disability?

Answer 3

1/50

Question 4

What is nuchal translucency measurement?

Answer 4

A test done 11 weeks to 13 weeks+6, or when crown rump length = 45-84mm. It has 85% detection for screen-positive rate of 5%

Question 5

Free β-hCG is _____ in chromosomally abnormal foetuses?

Answer 5

High

Question 6

What is the percentage of twins in pregnancy? How many are monozygotic and how many are dizygotic?

Answer 6

• Twins ~1% of pregnancies
o 2/3 dizygotic
o 1/3 monozygotic
o Increase in rate over recent times (aging population, IVF)

Question 7

What is NIPT?

Answer 7

A maternal blood test to determine whether the foetus has Down syndrome, at zero risk to the pregnancy. This works by testing foetal cell free DNA.
Very good for trisomy 21, 18, 13 and fair for sex chromosomes. Does not test other chromosomes. This now costs around $450.

Question 8

A form of invasive foetal assessment, what is CVS? What is Amniocentesis?

Answer 8

Chorionic villus sampling
• 11-14+ weeks
• 0.5-1% chance of miscarriage (greater with vaginal approach)

Amniocentesis
• 15-16 weeks onwards
• Ultrasounds guidance sample 15-20ml of amniotic fluid (containing skin and bladder cells)
• 0.5-1% miscarriage rate

Question 9

How many babies will have an anomaly when analysed under 2nd trimester anatomical assessment?

Answer 9

2-3%

Question 10

What are the 3rd trimester screening things?

Answer 10

• Growth measurement: biometry
• Amniotic fluid volume assessment
• Foetal wellbeing – Doppler blood flow studies
• Placental location: placenta previa