How much caffeine is it safe for pregnant women to consume? In the roughly 40 years of systematic research on this topic, the answer has shifted:
The established guideline is to Keep your intake below 300 milligrams a day, the figure recommended by the World Health Organization. This is around two 12-ounce cups of coffee.
The emerging general evaluation is more conservative: Be safe and make that 200 milligrams, avoid coffee but still enjoy the many health benefits of tea.
The new advice is: Avoid both tea and coffee as much as you can.
A trigger for the tightening of limits is a University College of Dublin report confirming earlier findings that even small amounts of caffeine lead to a reduction in the birthweight of babies and to smaller head circumference. The difference between high and low caffeine consumption is 6 ounces in newborn weight. Every 100 milligrams increase in intake in the first trimester is associated with a 2.5-ounce reduction in birthweight.
The relationship is causal. In addition, it is independent of source: coffee and tea have an equivalent effect.
It is unclear as to how much of a health risk this poses. There doesn’t seem to be a direct association between caffeine and miscarriage, heart defects, or other congenital health problems. That said, the figures bear consideration. (See Coffee & Caffeine During Pregnancy for a detailed summary of research.) Normal “gestational” weight is defined as 5.5 pounds to just under 9 pounds. About 10 percent of newborns fall below this range. The six-ounce high-low caffeine weight gap translates to 4 to 7 percent of normal and 15 percent of low-weight births.
The finding is not new in and of itself. A 1994 study reported on the caffeine-weight link, based on a review of more than 200 research articles. The discussion equates caffeine and coffee, with no mention anywhere of tea. In 2008, the UK National Health Service cut its recommended level of “moderate” intake from 300 to 200 milligrams, citing the weight issue as one of the main reasons.
The new 2018 study looked at almost 1,000 cases in Ireland, where tea drinking is higher than that of coffee. Half of the mothers drank tea and 40 percent drank coffee—the reverse of most countries. The striking new finding was that it made no difference whatsoever: caffeine is caffeine. It attributed the weight results to restricted blood flow in the placenta. The primary author of the report, Ling Wei-Chan, adds that caffeine passes into the placenta readily but clears more slowly as pregnancy progresses, accumulating in the fetal tissues. This leads to faster heart rates and risk of arrythmia. It reduces nutritional flows into the brain.
Commentators point out that the general population tends to associate caffeine with coffee—not tea—and underestimates daily consumption through sodas, chocolate, etc. There is a consensus that the 200/300 milligram limits are arbitrary and little more than convenient suggestions. One expert usefully summarizes the relevant studies: “The message to women I would prefer would be ‘the less the better.’ Then they can decide for themselves.”
Reuters: Caffeine in tea, coffee may be equally risky to fetus 11/22/18
American Journal of Clinical Nutrition: Caffeine linked to low weight babies, 10/19/18