Pregnant women who consume caffeine — even about a cup of coffee daily — are at higher risk of giving birth to an underweight baby, researchers said on Monday.

The new findings published in the British Medical Journal (BMJ) also linked any source of caffeine, including that from tea, cola, chocolate and some prescription drugs, to relatively slower fetal growth.

The findings are the latest in mounting evidence indicating the amount of caffeine a person consumes may directly impact one's health, especially when pregnant. In January, U.S. researchers found that pregnant women who drink two or more cups of coffee a day are at twice the risk of having a miscarriage as those women who avoid caffeine.

Babies born underweight are more likely to develop a range of health conditions when they grow older, including high blood pressure, diabetes and heart problems.

Women who drank one to two cups of coffee daily, or between 100-199 milligrams, had a 20 per cent increased risk of having a baby of low birth weight, the study found.

This was compared to women who consumed less than 100 milligrams daily.

"Caffeine consumption during pregnancy was associated with an increased risk of fetal growth restriction and this association continued throughout pregnancy," Justin Konje at the University of Leicester in Britain and colleagues wrote.

"Sensible advice would be to reduce caffeine intake before conception and throughout pregnancy."

Looked at 2,645 women

Konje and his team, which included researchers from the University of Leeds, looked at 2,645 women at an average age of 30 who were between eight and 12 weeks pregnant.

The women reported an average caffeine consumption during pregnancy of 159 milligrams per day, lower than new recommended limits of 200 milligrams in Britain.

The likelihood of having a low birth weight baby rose to 50 per cent for women who consumed between 200 milligrams and 299 milligrams each day, about two to three cups of coffee.

The impact was about the same as from alcohol and the association with low birth weight was maintained throughout a woman's pregnancy, the study found.

Even small amounts may prove harmful but Konje said in a telephone interview the best advice was to limit caffeine consumption to below 100 milligrams a day.

"We couldn't say that there was a lower limit for which there is no effect," he said. "My advice is if possible to reduce caffeine intake to a minimum. You have to be realistic because you can't ask people to stop taking caffeine."

Such studies don't prove that caffeine causes problems.

Dr. Gideon Koren, director of the Motherisk Program at Toronto's Hospital for Sick Children, has said that women need not stop drinking coffee, but should avoid overindulging.

For women of childbearing age, Health Canada's recommended maximum daily caffeine intake is no more than 300 milligrams.

With files from Reuters