Pregnant women will be warned this week to limit their caffeine consumption to two cups of coffee a day or risk giving birth to underweight babies.

The government’s food standards watchdog will issue guidance to coincide with a study linking caffeine to low birth weight. Scientists have previously linked it to miscarriages.

The advice from the Food Standards Agency (FSA) comes a week after scientists found that a weekly glass of wine during pregnancy could help boost a baby’s behaviour and vocabulary.

The FSA is lowering the current recommended caffeine limit of 300mg a day to 200mg, the equivalent of two average-sized mugs of instant or two cups of brewed coffee a day.

It has also identified other products containing caffeine that count towards the 200mg total. The limit is equivalent to four cups of tea, five cans of cola, three energy drinks or five bars of chocolate.

Andrew Wadge, chief scientist at the FSA, said: “This is new advice but these are not new risks. I want to reassure women that if you’re pregnant and have been following the previous advice, the risk is likely to be tiny.”

Coffee shops have higher levels of caffeine in their drinks than assumed in the FSA guide-lines, which state that an average mug of coffee contains 100mg.

A small cafe latte in Starbucks contains 240mg of caffeine, meaning a pregnant woman would exceed the recommended daily caffeine intake in one drink. A new generation of energy drinks, including Spike Shooter, contain up to 300mg of caffeine in a single can. The mass market drink, Red Bull, typically contains 80mg.

The FSA is changing its guide-lines following advice from the independent committee on toxicity, which after assessing the new research and previous studies decided that caffeine could be harmful to the unborn child at lower levels.

Researchers from Leicester and Leeds universities monitored the daily caffeine intake of 2,500 pregnant women using questionnaires. They compared this with the birth weight of the baby, taking into account the weight and ethnic background of the mother. The research, to be published in the British Medical Journal on Wednesday, found that women with a caffeine intake of more than 200mg a day were more likely to give birth to smaller babies.

One committee member said: “The [body of] research shows we get an adverse effect at a slightly lower caffeine intake than we previously thought, in terms of both reduced birth weight and increased instance of spontaneous abortion.

“The final decision mainly went on the birth weight with babies born at a weight appropriate for a baby a few weeks younger. If you’re small for gestational age, you’re more likely to have intellectual impairment and hyperactivity in later life.”

Research in the US published this year in the American Journal of Obstetrics & Gynaecology found that drinking more than 200mg of coffee a day doubled the risk of miscarriage.

Pat O’Brien, consultant obstetrician and spokesman for the Royal College of Obstetricians and Gynaecologists, said he recommended women should abstain from caffeine in the first 12 weeks of pregnancy. He said: “This is a very vulnerable time for the baby, and it’s when most miscarriages occur.”

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