Caffeine is the most commonly used psychoactive substance in the world. And at average consumption levels, it is considered to be good for your health – or at least not bad for your health. However, emerging evidence suggests that consuming caffeine during pregnancy may be bad for the baby. Our latest study adds to that evidence.

We wanted to know if there is a link between maternal caffeine intake and negative birth outcomes in a population where tea is the main caffeine source. To do so, we used data from an Irish cohort study. The data, on about 1,000 Irish women, provided us with the usual dietary intakes of caffeinated products during early pregnancy. We matched these with hospital records of the women's newborns to get information on the birth size and gestational age at birth.

Tea was the predominant caffeine source (48%) followed by coffee (39%). Our analysis, published in The American Journal of Clinical Nutrition, showed a consistent link between both coffee and tea caffeine and adverse birth outcomes. In the highest caffeine consumption group, the risks of delivering babies with abnormally low birth weight or short gestational age at birth were about two times higher. The results were similar regardless of the caffeine source.

Time to revise recommended intake levels?

While coffee is the main source of caffeine in most parts of the world (about 100mg per cup), it is less recognised that tea contains a significant amount of caffeine, too (about 33mg per cup).

Brewing methods and types of coffee and tea influence their caffeine contents. For example, caffeine content is higher in brewed coffee than in instant coffee, and it's higher in black tea than in green tea. So our findings have potentially important public health implications in countries where a lot of black tea is consumed, such as Ireland and the UK.

The recommended caffeine intake level during pregnancy differs across health organisations and countries. The World Health Organisation recommends an intake of less than 300mg per day. In contrast, organisations such as The Food Safety Authority of Ireland and The American College of Obstetricians and Gynecologists recommend an intake of less than 200mg per day. However, our study shows an increased risk of adverse birth outcomes even for a maternal consumption level lower than these recommended levels.

Should pregnant women abstain from caffeine?

Apart from smaller birth size and shorter gestational duration, maternal caffeine intake has been linked to other negative outcomes for the child, such as a lower IQ. But these findings, including our own, are from observational studies, and observational studies cannot prove that caffeine causes these outcomes, only that there is a link between them. To prove causation, scientists would need to conduct randomised controlled trials. However, to do so might be deemed ethically dubious.

Until more definitive evidence emerges, it is prudent to at least limit caffeine intake during pregnancy or when planning to conceive. It is not well-recognised that tea contains a significant amount of caffeine, so its consumption should also be taken into consideration when trying to adhere to the guideline caffeine intake limit during pregnancy.

Ling-Wei Chen is Postdoctoral Research Fellow in Nutritional and Life Course Epidemiology at University College Dublin. This article is republished from The Conversation under a Creative Commons license. Read the original article.