Consuming caffeine when pregnant may increase a woman's risk of having a baby with a low birth weight, a new study suggests.

Babies with a low birth weight are at an increased risk of short-term and lifelong health problems.

According to Swedish and Norwegian scientists, it is already well known that a mother's diet can play an important role in the health of her unborn child. However, along with oxygen and nutrients from food, caffeine is able to pass through the placenta into the baby.

The World Health Organisation currently recommends that pregnant women consume no more than 300mg of caffeine per day - that is two to four cups of brewed coffee.

The scientists decide to investigate the effect of maternal caffeine on unborn babies. They analysed almost 60,000 pregnancies. All sources of caffeine were studied, including caffeine from coffee, tea, fizzy drinks and certain foods.

The study found that caffeine from all sources lead to a reduced birth weight. For a child with an expected average weight, this equated to up to 28g lost per 100mg of caffeine consumed per day.

The study also found that pregnancies tended to be longer if caffeine was consumed, particularly if that caffeine came from coffee.

The length of a pregnancy increased by five hours per 100mg of caffeine consumed per day. This rose to eight hours if the caffeine came from coffee.

"Although caffeine consumption is strongly correlated with smoking, which is known to increase the risk for both preterm delivery and the baby being small for gestational age at birth (SGA), in this study we found no association between either total caffeine or coffee caffeine and preterm delivery, but we did find an association between caffeine and SGA.

"This association remained even when we looked only at non-smoking mothers, which implies that the caffeine itself is also having an effect on birth weight," the scientists explained.

As their results indicate that even 200-300mg of caffeine per day can increase the risk of a low birth weight baby, they called for a re-evaluation of these recommended daily intake levels.

Details of these findings are published in the journal, BMC Medicine.

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