Antenatal tests can be divided into two categories:

• Screening tests
• Diagnostic tests

Screening tests
Screening tests are usually done by taking a

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blood sample from the mother or by performing an ultrasound scan. The results can be given in two forms.

• A pregnant woman is given a “one in x” chance of having a baby with a particular condition. This figure means exactly what is says. If, for example, a certain feature is found in a baby which has a risk of one in 500 that the baby will be born with a particular medical condition, then this means that in a group of 500 babies with the same characteristics one will be affected and 499 will be normal. This is obviously a low risk but it is important to understand that the screening test cannot distinguish between the one affected baby and the other 499 unaffected ones. A risk of 1 in 5 still leaves you with a 80% (4 in 5) chance that the baby is not affected. Although this is certainly regarded as a high risk of abnormality (20%), the chances that the baby is normal are still four times greater.
• Another way of interpreting a test is in the form of a "positive" or "negative" result. This just means that doctors and laboratories have set a fixed number (e.g. 1 in 250) as the cut-off point for the risk of a certain condition. Any risk lower than this threshold is called "negative". Any risk higher than this is "positive". These terms may be confusing since a negative result does not mean for certain that a particular baby is not affected - it only means that the risk is lower than 1 in 250. On the other hand, a positive result does not indicate automatically that a baby must be abnormal - in fact the vast majority of babies in that risk group are absolutely normal.

There are different types of screening tests for the detection of a medical condition. These tests may, if compared with each other, have different detection rates regarding the number of affected babies they detect in a group of women who have an abnormal result.

Most private obstetricians offer some of these tests routinely to all pregnant women, while others only offer testing to older women.

Before embarking on the various tests, parents must realise that they have a 1 in 20 chance of being confronted with a worrying result and that they then have to face the decision regarding invasive diagnostic testing.

Diagnostic tests
Diagnostic tests give a conclusive answer about the presence of chromosomal abnormalities. However, they are invasive procedures that involve taking fluid or tissue from around the baby by introducing a needle into the uterus (womb). They can take weeks before giving a definitive answer, and all have in common a slight risk of leading to a miscarriage. - Health24
 

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