Prenatal diagnosis

Question 1

What are 2 ways to look for Down Syndrome during the first trimester?

What suggests Down syndrome in each case?

Answer 1

1. Ultrasound - increased nuchal translucency, lack of nasal bone
2. Serum markers - High hCG and low PAPP

Question 2

Compare the hCG, AFP, and inhibin levels in

Trisomy 18, Trisomy 21, and open neural tube defects

Answer 2

• Trisomy 18: low hCG, low AFP, low/nl inhibin
• Trisomy 21: high hCG, high inhibin, low everything else
• Open neural tube defect: only AFP is high

Question 3

What are two reasons for an elevated AFP?

Answer 3

gastroschisis or open NT defect

Question 4

What abnormalities can we detect with cell free DNA?

Answer 4

Common chromosomal abnormalities

(not single gene mutations)

Question 5

Describe how the PPV and NPV of a cell free DNA screen changes with age

Answer 5

• PPV increases with age
  
  • Positive result is much more likely to be true if mom is older
• NPV is high at all ages
  
  • Always feel confident in a negative test result

Question 6

Compare the 2 methods of diagnostic testing.

What is sampled in each and when can they be performed?

Answer 6

Chorionic villus sampling - sampling placenta, performed early (10-13 wks)

Amniocentisus - sampling fetal skin cells in amniotic fluid, performed later (after 15 weeks)