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Screening Tests

Timing of Prenatal Screening Tests

Timing prenatal screening FIRST TRIMESTER SCREENING (9 weeks to 14 weeks of pregnancy)

First trimester combined screening test

What is a first trimester combined test?

This screening test combines ultrasonography and maternal serum screening.

What do the results mean?

  • Results from a first trimester combined screen define the risk for your baby to have a birth defect or genetic condition.
  • If the risk is significantly greater than the average then a confirmatory diagnostic test is recommended, such as a CVS or amniocentesis.  

What are the limitations of the first trimester screen?

  • The test results do not conclusively confirm the presence of a birth defect or chromosomal abnormality.
  • There is a risk of a false positive result.
  • An abnormal first trimester screen may provoke anxiety.

What are the benefits of the first trimester screen?

  • The combined screen (ultrasonography and maternal serum screen) detects more than 90% of fetuses with Down syndrome.
  • It allows for early detection of an increased risk for chromosome abnormalities
  • If an increased risk is detected, an early diagnostic test (chorionic villus sampling) can be performed.
  • Early detection allows more time for counselling and decision-making in your pregnancy.
  • A negative result (no increased risk) is reassuring.

 

 

SECOND TRIMESTER SCREENING (16 to 23 weeks of pregnancy)

What is a second trimester combined screening test?

This test includes a maternal serum screen and ultrasonography.

What do the results from a second trimester screen mean?

  • Results from a second trimester screen test define the risk for your baby to have a birth defect or genetic condition.
  • Certain structural abnormalities are detected and diagnosed on ultrasound e.g. a neural tube defect
  • If the risk is significantly greater than the average then a confirmatory diagnostic test, an amniocentesis, will be offered.  

What are the benefits of a second trimester screen?

  • The combined screen (ultrasonography and maternal serum screen) detects more than 85% of fetuses with Down syndrome.
  • If an increased risk is detected, an early diagnostic test (amniocentesis) can be performed.
  • Early detection allows more time for counselling and decision-making in your pregnancy.
  • A negative result may result in reassurance about the pregnancy.

What are the limitations of a second trimester screen?

  • The test results do not conclusively confirm the presence of a birth defect or chromosomal abnormality.
  • There is a risk of a false positive result.
  • An abnormal screen may provoke anxiety.
  • An abnormal screen may provoke anxiety.
  • Diagnostic testing is performed later in pregnancy which may complicate decision making for the future of the pregnancy.
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