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Latest update: 28/06/2017

Prenatal examinations

  

Prenatal genetic tests allow us to identify mutations in DNA or abnormalities in fetal chromosomes in a targeted way. In the laboratory these analyses have a higher priority than postnatal tests due to their preventative nature. That is because if there is a severe abnormality the patient may decide to terminate the pregnancy or choose extra monitoring during pregnancy and after birth.

We carry out tests on material collected in various ways:

For monitoring purposes usually we carry out a test on the pregnant woman's blood sample and/or (possibly) a sample of her partner.

 

Depending on the expected abnormality, the samples are sent to the lab for:

 

Due to time pressure, prenatal molecular and biochemical screening are greatly targeted.

  • DNA tests are only carried out if the genetic defect or the condition has been identified in the parents. 
  • For instance, biochemical tests for a number of lysosomal accumulation diseases (see Biochemistry Laboratory) are only carried out for couples with a high recurrence risk: usually 25%, sometimes 50% for boys with X-bound lysosomal disorders.

 


    What does the combined test do?

    The combined test calculates the chances of a baby having Downs syndrome based on three factors:

    • your age (the mother's), 
    • the levels of two hormones in your blood (PAPP-A and free beta hCG),
    • three characteristics of the baby (nuchal skin fold, nasal bone and heart valve) which are visible via ultrasound

    As announced by Minister De Block, from 1st of July 2017 on, the combination test will only be performed after two failed Non-invasive prenatal tests (NIPT) .


    How does the test work?
    • The blood test to determine the hormone levels will only be performed after a second failed NIPT .
      The test takes a couple of days. 
    • The nuchal scan is performed during the first trimester ultrasound, between approximately 11 and 13 weeks of pregnancy. This ultrasound precisely measures the fluid under the skin in the baby's neck.
      In babies with Down syndrome — but also other hereditary and non-hereditary disorders — there may be an increased amount of fluid in the neck. The nuchal thickness is therefore not only an indicator for Down syndrome, but also for other abnormalities. 
    • We also check the calcification of the nasal bone and the operation of the heart valve.
      Babies with trisomy 21 have a slower calcification of the nasal bone and often a leaky heart valve.

    The combination of age, blood values and ultrasound characteristics results in a particular risk figure. The risk is higher if this figure is equal to or greater than 1 out of 300 (e.g. 1 out of 110).

    How reliable is the combined test?
    • The combined test has a sensitivity of 80 to 85%: of 100 babies with trisomy 21 the test will detect approximately 85 and miss 15.
    • In five percent the combined test gives a false positive. This means that when a hundred women have this test, five women will show to have an increased risk whereas the baby does not have trisomy 21.
    What are the risks for the baby?
    • In itself, the combined test is a non-invasive screening test. The blood test and ultrasound hold no risk for the pregnancy.
    • As the test can show a false positive in five percent of women, this means that you have one chance out of twenty of having an invasive test, with an increased risk for the baby, whereas it does not have trisomy 21.
    When is the result of the combined test available?
    • If blood can be taken beforehand, the person performing the ultrasound can immediately calculate the risk and tell you.
    • If blood is taken on the day of the ultrasound and if your result is abnormal, you will be notified within a week by your own gynaecologist or the person who performed the ultrasound.
    How sensitive is the combined test?
    • The combined test's sensitivity is approximately 80 to 85%: out of 100 babies with trisomy 21 the test will detect approximately 85 and miss 15.
    • In 5% the combined test gives a false positive. This means that when 100 women have this test, 5 women will show to have an increased risk whereas the baby does not have trisomy 21.
    How much does the combined test cost?
    The combined test is from 01-07-2017 onwards only reimbursed by the national health insurance after a second failed NIPT. You pay the amount of the standard pregnancy ultrasound.
    If you don't have Belgian medical insurance, the blood test costs approximately EUR 25.