Pregnant women should keep consumption of coffee, tea and cola to a minimum, according to a new study that sees a link between caffeine intake and low birthweight among babies.

British researchers recruited 2,635 women at between eight and 12 weeks of pregnancy, asked them about their dietary habits and monitored them for caffeine in their saliva at checkups.

Research has found that caffeine easily crosses the placenta into the foetus.  

Women who took between 100 and 199 milligrams of caffeine per day faced a 20 per cent increased risk of having a baby with a relatively lower birth weight compared with counterparts whose intake was less than 100mg per day.

For those who had an intake of 200-299mg per day, the risk rose to 40 per cent, and for those over 300mg per day, it was 50 per cent.

Birthweight is a widely used benchmark of health, especially for diabetes and heart disease.

An average cup of coffee contains about 100mg of caffeine and tea about half this amount, although the concentration varies according to the strength of brew and the brand of the product.

Caffeine is also present in cola, chocolate, coca and some drugs.

The so-called CARE study, published online by the British Medical Journal, said "sensible advice" would be for women to reduce caffeine intake before conception and throughout pregnancy.

Previous research has found that caffeine easily crosses the placenta into the foetus, although the evidence for low birthweight has also been inconclusive and muddied by factors such as smoking and alcohol consumption.

This is the first study that also sees a link in tea, which comprised 60 per cent of the caffeine consumed by the women.

On its website, Britain's Food Standards Agency (FSA) said it was cutting its recommended maximum intake of caffeine for pregnant women to 200mg per day - the rough equivalent of two mugs of coffee - from 300mg in light of the study.

"This new advice doesn't mean that pregnant women have to cut out caffeine completely, simply that they should be careful and make sure they don't have too much. We would emphasise that the risks are likely to be very small," said the FSA's chief scientist, Andrew Wadge.

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