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 United States, 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.