The Food Standards Agency (FSA) revised its guidance from a maximum of 300mg of caffeine a day to 200mg, a reduction of one third.
It now means women should stick to a maximum of two average-sized cups of coffee per day although women who have been drinking three cups have been told not to panic.
The FSA's chief scientist Andrew Wadge 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 small."
Children born with a low birth weight are more likely to suffer a range of health problems including heart disease and diabetes.
The FSA guidance means women should limit themselves to three or four cups of tea a day and watch their intake of cola, chocolate and energy drinks as all contain caffeine.
Drinks bought from coffee shops may be far higher than the FSA estimates for the average-sized home-made cup of tea or coffee.
The change in guidance follows new research from Leicester and Leeds universities.
The study, which involved 2,500 pregnant women, was be published in the British Medical Journal.
The revised guidance on caffeine comes only days after a study suggested drinking lightly during pregnancy reduced the risk of problem behavior and hyperactivity in boys.
Current medical advice is that women should avoid alcohol while pregnant or trying to conceive.
The University College London study found boys born to light drinkers did better on vocabulary tests than those born to mothers who abstained completely during pregnancy.
Girls were less likely to have emotional symptoms and peer problems if their mother drank lightly -- one or two units of alcohol a week -- during pregnancy although this appeared partially explained by family and social backgrounds.
Earlier this year, scientists in the U.S. published research which found 200mg of caffeine a day doubled the risk of miscarriageor nationwide distribution by mid-2009.
From the November 10, 2008, Prepared Foods e-Flash