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Apart from maternal age, are there any other risk factors?
Yes. One or the other parent may carry a potential chromosomal abnormality, even though the parent is normal. Through a complicated chromosome material swap (known as "translocation", in medical language), the parent may produce gametes with extra chromosomal material. If this extra chromosome is on number 21, the resulting fetus will be Down's. If it is the father who is carrying the translocation, the risk of producing a Down's baby is one in ten (10 per cent).- If it is the mother who has the translocation, the risk is much higher at one in two (50 per cent). This kind of risk is independent of the age of either parent.
What proportion of Down's syndrome children result from parental chromosomal translocation?
About 2 per cent. When a Down's baby is born to a young mother, it may be worthwhile to check the chromosomes of the parents to establish whether one of them might be a carrier of a translocation. This will help in counselling regarding future pregnancies and estimation of risk of recurrence: Remember, the carrier parent will have no telltale features of any kind.
Talking of recurrence, what is the risk of this happening?
Except for those carrying a translocation, a mother who had a Down's pregnancy has a risk of this recurring increased three-fold compared to the average. This means, if the mother is going to be thirty at her next pregnancy, while the average risk is 1 in 900 at this age, her risk will be significantly higher at 1 in 300.
What are the screening tests for Down's syndrome?
Any woman who has been pregnant will be aware of the terms "triple test" or "double test". What the triple test really means is that a small sample of blood taken from the mother is checked for three (hence "triple") different hormones produced by the fetus or placenta. The levels are computed, together with other important factors such as maternal age, gestational age and previous history, to estimate the risk in that particular pregnancy. In the double test, only two of the hormones are checked. The sensitivity of the "double" test in detecting an affected baby is similar to that of the "triple" test. Maternity units tend to adopt one or the other.
Why are those particular 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 (αFP), which tends to be reduced in Down's syndrome, and beta-hCG (ßhCG), which tends to be increased in Down's.
Is there any particular gestational age when the screening test has to be done?
Yes. The screening test is accurate if the blood is taken between fifteen and seventeen 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.
So, what can the mother do if she is unsure about the gestational age?
An ultrasound scan to confirm or establish the gestational age is easily arranged. The scan in early pregnancy is pretty accurate, with a margin of error of only about three or four days.