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Diagnostic Testing

Diagnostic testing is the only way to determine with certainty if the baby has a chromosome problem, such as Down syndrome or trisomy 18.

Currently, pregnant women are eligible for amniocentesis or CVS if they have:
  • a positive prenatal screening test
  • abnormal ultrasound findgins
  • a family history of genetic disease or are known chromosome rearrangement carriers
  • prengancy conceived by IVF with intracytoplasmic sperm injection
It is important to know that each centre has their own eligibility criteria for diagnostic testing.  Some centres offer diagnostic testing to patients depending upon their age.  The SOGC has recently published guidelines that recommend no age based diagnostic testing.   (2011 SOGC Screening for Fetal Aneuploidy)
Prenatal diagnostic testing consists of chorionic villus sampling (CVS) and amniocentesis.

CVS

Amniocentesis

Performed

11-13 weeks1

15-17 weeks (ideal) but available up to 22 weeks2

Sample

Placental villi

Amniotic fluid

Results available

2-3 weeks

2-3 weeks

Miscarriage rate

1%

0.01 – 0.5%3

Advantage
– Accurate
– 1st trimester test – earlier results
– Accurate
– Widely available
– Tests for NTDs
Disadvantage
– Availability varies
– Does not test for NTDs
– ↑ rate of repeat procedures due to ambiguous results

– 2nd trimester test – later results

1.  The timing of CVS may vary between centres.
2.  Amniocentesis may be available later than 22 weeks in certain circumstances.
3.  Recent studies suggest that miscarriage rate is lower than 1 in 200 (0.5%).