One cup of fresh coffee a day significantly increases the chances of giving birth to an underweight baby, research has shown.
The new findings from a large Scandinavian study suggest current UK guidelines on caffeine consumption during pregnancy may not go far enough.
A daily dose of between 200 and 300 milligrams of caffeine - roughly equivalent to one cup of fresh coffee or two cups of instant - increased the risk of a baby being small for gestational age (SGA) by up to 62%.
The association was independent of smoking, a major risk factor for low birth weight which may be linked to caffeine intake.
Coffee, but not other sources of caffeine, was also associated with longer pregnancies.
The reason for this remains a mystery. It could be due to some aspect of behaviour rather than the effects of any substance in coffee, according to the researchers.
Currently the World Health Organisation recommends a caffeine consumption limit of 300 milligrams per day for pregnant women. In the UK, Nordic countries and the US, the limit is set lower at 200 milligrams.
However, the new study shows that even 200 milligrams per day of caffeine can increase SGA risk. Writing in the online journal BMC Medicine, the researchers concluded: "This association should be further investigated and recommendations might have to be re-evaluated."
A baby is said to be small for gestational weight if it weighs 2.5 kilograms or less after a normal length pregnancy.
Such babies can suffer complications at birth or long term health problems, including impaired neurodevelopment.
Along with nutrients and oxygen, caffeine passes across the placental barrier but is not easily inactivated by the developing foetus.
To investigate the impact of caffeine intake during pregnancy, the scientists collected information about diet and birth outcomes for 59,123 pregnant women.
The effects of higher caffeine consumption were compared with those of a low intake of 50 milligrams per day.
Findings were based on three different methods of analysing foetal growth. They showed that a caffeine intake of 200 - 300 milligrams raised the risk of giving birth to an SGA baby by 27%, 44% and 62% depending on the method used.
Women were asked to list all their sources of caffeine, including coffee, tea, fizzy drinks, cakes, deserts and chocolate.
Caffeine from any source was found to reduce birth weight. For an average child expected to weigh 3.6 kilograms its impact equated to a loss of 21-28 grams per 100 milligrams of caffeine consumed a day.
Lead scientist Dr Verena Sengpiel, from Sahlgrenska University Hospital in Sweden, said: "Although caffeine consumption is strongly correlated with smoking which is known to increase the risk for both pre-term 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 pre-term 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."
For reasons that are still unclear, caffeine from coffee alone was associated with a pregnancy lengthening of eight hours for every daily dose of 100 milligrams.
"Whether it is some other substance present in coffee, but not in other caffeine sources, or whether coffee drinking is associated, on a behavioural level, with some factor influencing gestational length remains to be investigated," the researchers wrote.
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