Antenatal tests for twins

It’s more difficult to test twins for a condition such as Down syndrome, as the results of a single blood test can’t be applied to two babies.

In this situation you’ll have a nuchal translucency scan as part of your first trimester screen, between weeks 11 and 13, and your sonographer will be able to examine both of your babies for signs of Down syndrome. Keep in mind that the nuchal scan is a screening test and can only assess your risk factor, not offer a diagnosis. If there are any concerns over one or both of your twins, you may be offered a test called chorionic villus sampling (CVS), which can be carried out between weeks 10 and 13; or an amniocentisis between weeks 14 and 20.

On rare occasions, twin babies share a membrane sac but if they each have their own sac it’s possible to take a sample of fluid from each during the amniocentesis, in order to test both babies. With more than two babies it may not be possible to collect a sample of cells from each baby. These tests are invasive and carry a risk of miscarriage which is higher for twins and multiples than for single babies.

Testing two babies for a chromosomal condition could be problematic, if you were to discover that one of your babies was positive and the other negative. You’d have to carefully consider what you would do in that situation, as terminating one baby (which is possible) may be especially traumatic for all those involved. Selective termination, as it’s called, is the process by which one or more of multiple babies is aborted. There’s a small risk that the healthy baby may be affected by the procedure and that both may miscarry.

Arm yourself with facts when considering this sort of procedure and make sure you fully understand all the possible outcomes by talking through your concerns and feelings with your OB-GYN or a genetic counselor beforehand.

The information in this feature is intended for educational purposes only. If you have any concerns about your health, the health of your child or the health of someone you know, please consult with a doctor or other healthcare professional.

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Last Modified: 21/06/2007

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