Pregnant women in UK are being told that more than two cups of coffee a day could mean underweight babies.
The country's Food Standards Agency has lowered the limit from 300milligrams a day to 200mg a day.
That works out to two mugs of instant coffee, two cups of brewed coffee, four cups of tea, five cans of cola, three energy drinks or five bars of chocolate.
But some coffee shops have much higher levels of caffeine in their drinks - such as a small cafe latte in Starbucks which contains 240mg. A single cup would exceed the new daily limit.
It comes a week after scientists said light drinking during pregnancy can make a child more intelligent and better behaved - despite Government advice saying mothers to be should not drink at all.
Children born underweight are more likely to die early, or suffer developmental problems.
Britain has the second worst rate for low birth weight babies in western Europe.
In 2006, 78 out of every 10000 babies were born at less than 5lb 8oz, compared to 67 in 1989. This is 40,000 babies in England alone.
The FSA said it was acting after following advice from the independent committee on toxicity, which suggested that the advice should be changed after looking at new research from Leeds and Leicester universities.
Scientists gave questionnaires to 2,500 pregnant women asking them about their daily caffeine intake, and then compared this with the birth weight of the baby, taking into account the weight and ethnic background of the mother.
The study, which will 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.
A member of the toxicity committee 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.'
It follows US research earlier this year which found that drinking more than 200mg of coffee a day doubled the risk of miscarriage.
Pat O'Brien, of the Royal College of Obstetricians and Gynaecologists, said 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.'
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.'