A new study from the British Medical Journal says pregnant women who consume caffeine -- even about a cup of coffee daily -- are at higher risk of giving birth to an underweight baby.
The new findings also linked all sources of caffeine, including that from tea, cola, chocolate and some prescription drugs, to relatively slower fetal growth.
They are the latest in mounting evidence indicating that caffeine intake directly affects a person's health, especially during pregnancy.
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.
According to the study, women who drank one to two cups of coffee daily, or between 100-199 milligrams, had a 20 percent increased risk of having a baby of low birth weight. 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 Britain's University of Leicester wrote.
He advised that women reduce caffeine before conception and throughout pregnancy."
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 8 and 12 weeks pregnant.
The women reported an average caffeine consumption during pregnancy of 159 milligrams per day.
The likelihood of having a low birth weight baby rose to 50 percent 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.
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."