Women who drink moderate amounts of coffee while pregnant should be reassured they are not increasing risks to their baby, a Danish study suggests.
Pregnant women are advised to have no more than three cups of coffee a day
Earlier work found high caffeine intake could increase the risk of premature birth and having a small baby.
But the British Medical Journal research found no difference between women who drank moderate amounts of caffeinated and decaffeinated coffee.
UK pregnant women are advised to drink no more than 300mg of caffeine per day.
This equates to the amount of caffeine in three cups of instant coffee, six cups of tea, eight cans of cola or eight bars of chocolate.
The Food Standards Agency advises having a bar of plain chocolate, drinking three cups of tea, a can of cola and a cup of instant coffee in a day would meet the 300mg limit.
The researchers from the University of Aarhus monitored 1,207 healthy pregnant women who had what was classed as a high caffeine intake of more than three cups of coffee a day.
All were less than 20 weeks pregnant when they joined the study.
Half the women were given normal coffee and half decaffeinated coffee so researchers could evaluate who was most at risk of giving birth early or of having a small baby.
Neither the women, nor the researchers who gave out the coffee knew which sort was being given.
Each woman was also interviewed to check if they were drinking other drinks containing caffeine, such as tea and cola.
The length of pregnancies and the baby's birthweight was also recorded.
Age, pre-pregnancy weight and smoking habits were also taken into consideration.
It was found that there was no real difference in either the length of pregnancy or birth weight between the two groups.
Women who had the decaffeinated coffee had a caffeine intake which was 182g lower than those who drank the caffeinated form.
Among women who drank the caffeinated coffee, 4.2% of babies were born prematurely and 4.5% were small for their gestational age.
In the group drinking decaffeinated coffee, 5.2% of babies were born prematurely and 4.7% were underweight.
There was virtually no difference between the average birthweights in each group.
The researchers said decreasing caffeine intake during the later stages of pregnancy has no overall effect on birth weight and length of pregnancy.
However, they said any effect in the first half of pregnancy would not have been detected by their study.
'Don't cut it out'
Dr Bodil Hammer Bech, who led the team, said: "We found no overall effect of moderate caffeine intake on birthweight or gestational age."
"However, in secondary analyses we found that caffeine may lower birthweight in smokers."
She added: "I don't think a moderate intake of caffeine harms the foetus. Thus if a pregnant woman likes caffeinated coffee, I think she should enjoy a cup!"
A spokeswoman for the Food Standards Agency said it would consider the paper in detail.
She added: "These results are consistent with the current FSA advice, which is that pregnant women should not exceed an intake of 300 mg/day caffeine (approximately three mugs of instant coffee).
"The Food Standards Agency has also commissioned further research addressing the health effects of caffeine during pregnancy. This project is due to report in summer 2007."
Sue Macdonald, education and research manager at the Royal College of Midwives said: "This is a very interesting study, and may be helpful in providing more information for pregnant women regarding lifestyles and healthy pregnancies.
She advised women to always ask their midwife for the latest advice on issues relating to what they can eat and drink while pregnant, adding "Ensuring adequate intake of water, juice and/or herbal teas is considered better than too much coffee, tea and carbonated, and in particular sugary, drinks."