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Hormones checked to screen for Down's syndrome

It is true that the fetus and the placenta produce many different types of hormones and other chemicals. It is, however, known that some hormones tend to be increased and others reduced in babies with Down's syndrome. By computing levels, therefore, one can have a pretty good impression of which babies are likely to be affected.

Two of the most important chemicals are AFP (áFP), which tends to be reduced in Down's syndrome, and beta-hCG (ßhCG), which tends to be increased in Down's.

This screening test is most accurate if the blood is taken between 15 and 17 weeks of gestation. It is, therefore, important to be very accurate about the gestation. If the dating of the pregnancy is wrong, this will corrupt the calculation of the risk, either making it too high or too low - with all the inevitable consequences.

If the mother is unsure about gestational age, an ultrasound scan to establish the exact gestational age is arranged. The scan in early pregnancy is pretty accurate, with a margin of error of only about three or four days.

Sensitivity of Triple Test

With the ‘Triple screening Test’, about 60% of all affected babies will be detected which leaves 4 out of 10 undetected by this standard screening test. It is clearly unsatisfactory. Other, more sensitive tests are already available but not yet universally adopted for a variety of reasons.

For the babies that slip through this test, there is still a possibility that an affected fetus can be detected using the detailed anatomy ultrasound scan, usually performed at 18 to 20 weeks of gestation. As mentioned before, Down's babies tend to have a number of physical abnormalities and organ malformations, which may be detectable by the scan.

If a number of those anomalies are detected, in spite of a "negative" screening test, the suspicion may be aroused to justify suggesting a definitive diagnostic test. As it happens, not all Down's babies have these abnormalities and therefore a substantial proportion still slip through and are born in spite of a ‘negative’ screening test and normal anatomy scan.

Definitive diagnostic tests for Down’s syndrome

For all chromosomal disorders, including Down's syndrome, a amniocentesis confirmatory diagnostic test is based on obtaining fetal (or placental) cells which are processed to enable the mapping of the chromosomal make-up of that baby. Once this is achieved, a chromosomal defect, be it an extra chromosome (as in Down's) or deletion of part or a whole chromosome, will be established.

There are three main ways of getting the fetal cells. The most widely used is amniocentesis, where fluid is obtained from around the fetus and fetal cells isolated from this and then cultured. The second method is chorionic villus sampling (CVS), where a tiny piece of placenta is obtained; and the third is cordocentesis, where fetal blood is obtained from the umbilical cord. All these tests are invasive.

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