"Based on the consistent associations we observed, and because many pregnancies are unplanned, we would recommend women who are pregnant or seeking to become pregnant to at least limit their intakes of caffeinated coffee and tea," lead study author Ling-Wei Chen, a researcher at University College Dublin in Ireland, said in an email.

The study cannot prove cause and effect, he cautioned. But several previous studies have linked caffeine exposure in the womb to negative effects, Chen and his colleagues write in The American Journal of Clinical Nutrition.

In most of those studies, coffee was the main caffeinated beverage consumed.

For the current study, Chen's team looked at 941 mother-child pairs born in Ireland, where people drink more tea than coffee. Nearly half of the mothers in the study drank tea, while about 40 percent drank coffee.

Every additional 100 milligrams of caffeine consumed daily during the first trimester of pregnancy was associated with a 72-gram (2.5 ounce) lower birth weight, as well as significantly lower gestational age, birth length and head circumference.

Women who took in the most caffeine had babies weighing about 170 grams (6 ounces) less than those who consumed the least, the researchers found. Whether the caffeine came from coffee or tea made no difference to the results.

"High caffeine intake can result in restricted blood flow in the placenta which may subsequently affect fetal growth," Chen explained. "Caffeine can also cross the placenta readily, and because caffeine clearance slows as pregnancy progresses, caffeine accumulation may occur in fetal tissues."

One 12-ounce cup (355 ml) of brewed coffee contains about 200 mg of caffeine, but tea generally contains less caffeine and the amount can vary by type of tea and how long it was brewed. The Lipton Tea company, for example, says its black tea contains about 83 mg caffeine in a 12-ounce cup.

The public doesn't seem to recognize tea's caffeine content, said the study's senior author, Catherine Phillips, a researcher at University College Dublin. "Thus, maternal intake of both tea and coffee need to be taken into account when trying to minimise overall caffeine intake during pregnancy. This is especially important where tea is the predominant caffeine source," she told Reuters Health by email.

ACOG states that consuming less than 200 milligrams of caffeine per day in pregnancy "does not appear to be a major contributing factor in miscarriage or preterm birth." The World Health Organization recommends that women consume less than 300 mg of caffeine daily in pregnancy.

"This paper and other papers show that this is likely too high," said Dr. De-Kun Li, a senior scientist at the Kaiser Permanente Northern California division of research in Oakland, who wasn't involved in the study. "Epidemiological findings based on self-reported caffeine consumption are usually not very precise. Thus, any cutoffs chosen by ACOG and WHO can only be considered as rough reference points. Biologically, it is unlikely that 300 mg is risky while 299 mg is safe," Li said in a phone interview.

"The message to women I would prefer would be 'the less the better,'" Li said. "Then the women can choose for themselves."

Reports touting the health benefits of caffeine also need to include a caveat stating that it may be harmful in pregnancy, added Li, whose own research has linked caffeine consumption in pregnancy to miscarriage and childhood obesity. "My advice would be try to reduce as much as you can, if you can totally quit that would be even better."

ACOG told Reuters Health that it reviews all of its recommendations every 18 to 24 months, includes all new research in its reviews, and makes adjustments to recommendations as needed. "Until that time, ACOG's current guidance still stands."