A study conducted by British researchers on 2645 pregnant women, with average age 30 and with low risk pregnancies revealed that even low amount of caffeine could lead to low weight babies. 

It was found that the average caffeine intake during pregnancy was 159 milligrams a day which is much lower than the 300 mg/day recommended by the U.K.'s Food Standards Agency. Tea was the main source of caffeine (62 %), followed by coffee (14%), cola (12%), chocolate (8%) and soft drinks (2%).

Women whose intake of caffeine was between 100 and 199 milligrams per day faced 20% percent increased risk of having a baby with a low birth weight as compared to whose intake was less than 100 mg per day.

The risk increased to 40% for those who had an intake of 200-299 mg per day. Those women who had over 300 mg per day, it was 50 percent.

It was also demonstrated that Caffeine consumption of more than 100 mg/day could lead to fetal weight reduction of 34-59 grams in the first trimester, 24-74g in the second trimester, and 66-89g in the third trimester. The effect of caffeine was more prominent as low birth weight in fetus in women who metabolized caffeine more quickly.

Britain's Food Standards Agency (FSA) said it was decreasing the recommended maximum intake of caffeine for pregnant women to 200 mg per day from 300 mg in the light of the study. This means roughly two mugs of coffee.

"This new advice doesn't mean that pregnant women have to cut out caffeine completely, simply that they should be careful and make sure they don't have too much. We would emphasise that the risks are likely to be very small," said the FSA's chief scientist, Andrew Wadge.