Previous research has shown that caffeine consumption during pregnancy could reduce birthweight, but the levels of caffeine associated with the effect were unclear.
This latest study found that consuming more than 200mg of caffeine daily, equivalent to two cups of coffee or four cups of tea, in pregnancy could increase the risk of fetal growth restriction.
In light of the findings, the Food Standards Agency (FSA) has lowered the recommended caffeine limit for pregnant women from 300mg to 200mg.
FSA guidance should reassure women about safe levels of caffeine
What is the research?
The findings are based on a study of 2,645 pregnant women with an average age of 30, who were eight to 12 weeks pregnant.
Each woman was asked to complete a questionnaire about their caffeine intake for the period starting four weeks before their pregnancy and then throughout pregnancy.
Once the participants' babies were born, the researchers analysed data on the length of the pregnancy, birthweight and gender. They found 13 per cent of women gave birth to babies with fetal growth restriction.
After adjusting for other risk factors for low birthweight such as smoking and alcohol consumption, mothers who drank more than 200mg of caffeine daily had a 40 to 50 per cent greater risk of giving birth to a baby with a low birthweight compared with mothers who drank less than 100mg a day.
Mothers who consumed more than 200mg of caffeine a day had babies that weighed about 60-70g less than those who consumed less than 100mg daily.
What do the researchers say?
Lead researcher Dr Marcus Cooke, from the University of Leicester, said: 'This significant study has addressed many of the shortcomings which exist in the literature, providing clarity concerning the issue of caffeine consumption in pregnancy.
'These findings should be seen not as more bad news for prospective mothers, but as robust data upon which informed decisions can be made, providing reassurance for mothers.'
Professor Justin Konje, chairman of the project steering group, from the University of Leicester, added that tea was the major source of caffeine for most of the women in the study.
'It is important for expectant mothers to understand that caffeine is not just in coffee and cola,' he said.
What do other researchers say?
Andrew Wadge, chief scientist at the FSA, said: 'This new advice does not mean that pregnant women have to cut out caffeine completely, simply that they should be careful and make sure they do not have too much.
'We would emphasise that the risks are likely to be very small and believe our new advice, which is based on new research and has been considered by leading independent scientists, is sensible and proportionate.'
- Consuming large amounts of caffeine in pregnancy could increase the risk of fetal growth restriction.
- The association for caffeine intake with fetal growth restriction is similar to that for alcohol intake.
- FSA guidelines say that pregnant women should limit their caffeine intake to 200mg a day.