Estimating risk of Down's syndrome in a twin pregnancy

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. In such a case, 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.

A situation where there is suspicion that one of the twins might be affected poses a difficult challenge. 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.

Selective termination of pregnancy

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 in the process. The risk of this is of the order of about 15%, which is quite considerable. In the UK, this service is available only in specialist centers.

If, either by intuition or for some other reason, the parents seek a diagnostic test in the earlier stages of pregnancy - let's say 11 – 13 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%. The only risk-free strategy is to opt to continue with the pregnancy as it is.

Early screening test for Down’s syndrome

We have discussed the Integrated Test (above), an integral part of which is The Nuchal Translucency Scan Test. This scan needs to be done in a nuchal translucency scan narrow window at the end of the first trimester (11-13 weeks).

It involves measuring the thickness of the back of the fetal neck using an ultrasound. The principle is based on the finding that babies with Down’s syndrome and other chromosomal anomalies tend to have increased thickness of this part of the body (nuchal pad) at this stage of the pregnancy.

When the nuchal pad is found to measure more than 3 mm, the suspicion is aroused. When the thickening is much more than this, the possibility of a chromosomal anomaly is very high as we will see shortly.

Sensitivity of nuchal translucency scan

Results so far indicate that the nuchal translucency scan may have a sensitivity of between 65 and 80%, comparing favorably with the established screening (blood) tests. Moreover, it is claimed, if the nuchal pad thickness is more than 6 mm, the possibility of an abnormality (Down's or otherwise) is well over 90%. This scan is now part of the screening ‘integrated test’ mentioned earlier.

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