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What is the miscarriage rate following amniocentesis?
On average; the quoted figure is 0.5 per cent. This means one in 200 mothers who undergo the procedure will miscarry as a consequence. However, latest studies indicate that procedure-related miscarriage rate may actually be a lot lower than this. An authoritative study concluded in 2006 quoted a figure of 0.06% which is almost 10 times lower than the traditionally quoted figure. In any case, sadly, some of the babies lost do not have chromosomal defects. It is up to the mother and her partner to decide whether the risk is worth taking.
What is to be done if the diagnosis is confirmed?
It is again up to the parents to decide what to do. In fact, this is an area that should be sorted out well before even the amniocentesis has taken place. Many parents of affected fetuses opt for termination of pregnancy. Others opt to continue with the pregnancy and value the forewarning which prepares them to look after a handicapped child.
In the case of twin pregnancy, what effect does this have on the screening tests?
There are some unique problems here. If the screening blood test is performed before the twin nature of the pregnancy is recognised, as is occasionally the case, the result may be misinterpreted.
The error is recognised with an ultrasound scan, which will establish the status and which is the next test performed in case of any abnormal result. The results are usually recalculated in the light of the new information to take into account the presence of more than one fetus in the womb.
Is the risk of Down's syndrome increased in case of twin pregnancy?
It depends. If the twins are identical, the risk is the same as for a singleton. This is because the chromosomal constitution of the two fetuses is similar. This means, either both will be normal or both will be affected. If the twins are non-identical, the risk is twice that of the norm. This is because the fetuses are two distinct individuals and either of them (or, rarely, both!) can be affected.
If there is a suspicion that one of the twins is affected, how easy is it to verify this?
It is a difficult situation. It requires taking a sample of fluid from each sac and identifying with certainty, which sample came from which sac. If one of the fetuses is confirmed to be affected, the parents may opt to have selective termination of the affected fetus.
How easy is it to do selective termination?
Technically, this is not difficult. However, it means doctors have to be absolutely certain they are terminating the affected fetus. This is not always easy. Moreover, there is the additional concern that by selectively terminating one, the healthy twin may also be lost. The risk of this is of the order of about 15 per cent, which is quite considerable. In the UK, this service is available only in specialist centres.
Is there any way round this problem of loss of the healthy twin?
Not entirely. However if, for some reason, the parents seek a diagnostic test in the earlier stages of pregnancy - let's say eleven to thirteen weeks, where a CVS will be used - then if selective termination becomes necessary, the risk of losing the healthy twin is a comparatively lower at 5 per cent. The only risk-free strategy is to opt to continue with the pregnancy as it is.