They say an ultrasound technique can identify heart defects in unborn babies from 10 weeks into pregnancy, allowing early referral for specialist treatment.

A study tested the technique in 29,154 pregnancies and found it could reveal 55% of major defects of the heart and arteries between 10 and 14 weeks of pregnancy.

Current methods identify 26% of major defects and are used at 20 weeks.

Cardiac defects are the most common abnormalities with which babies are born, affecting 3.8 per 1,000 pregnancies in the UK.

Half of these are harmless, but the other half can be fatal or require surgery.

Sensitivity

The study authors said using ultrasound at an earlier stage could determine which ones require treatment.

The method involves scanning the thickness of fluid accumulating behind the neck of the foetus rather than examining the heart itself.

"Extensive studies" have shown a link between cardiac defects and this thickness, called the nuchal translucency thickness, the researchers said.

The more fluid that accumulates, the greater the risk of an abnormality.

About 90% of foetuses with a nuchal translucency thickness of 3mm at 12 weeks old will be born without problems, while 10% may have defects.

Greater risks

With double the thickness the situation is reversed - only 10% of foetuses with a measurement of 6mm at 12 weeks will be born normal.

There is an opportunity to detect abnormal build-up of fluid between 10 and 14 weeks old, but after that the baby's system is likely to be well-developed enough to drain excess fluid and disguise any abnormality.