Caffeine consumption is one of the many things that may require adjusting when someone becomes pregnant.

Unfortunately, years of conflicting research have made the advice confusing, and even among scientists there's some disagreement over whether the current guidelines are appropriate. A new study on caffeine's impact on children's brain development has now added to the conversation.

When caffeine is consumed during pregnancy, it's known to cross the placenta. What's more, as pregnancy progresses, the mother's body slows down in its metabolising of the drug; whether this might result in any short- or long-term effects on the health of the child as they grow up remains hotly contested.

Currently, the American College of Obstetricians and Gynecologists and the American Pregnancy Association suggest people who are pregnant should consume less than 200 mg of caffeine per day - what essentially amounts to one to two cups of coffee, depending on strength (although most pregnant people drink less than this).

The UK National Health Service (NHS) and the European Food Safety Authority (EFSA) also advises limiting caffeine to a moderate amount of 200 mg whilst pregnant. The World Health Organisation defines a high caffeine intake in pregnancy as anything over 300 mg.

The thresholds are pretty much all in agreement, and yet recently there have been several published studies that have raised concerns about current guidelines.

In 2020, a narrative review of past research found there was no safe level of caffeine intake during pregnancy or even while trying to conceive.

The findings made headlines, but other experts have since called the conclusions 'alarmist'. They argue this one study doesn't supersede all the other evidence that a limited amount of caffeine is safe in the majority of pregnancies.

Obstetrician Daghni Rajasingham, a spokesperson for the Royal College of Obstetricians and Gynaecologists, said the results of this review agree with a large body of evidence that pregnant people should limit caffeine intake during pregnancy, but that "pregnant women do not need to completely cut out caffeine."

While past studies have shown high levels of caffeine during pregnancy are linked to miscarriage and low birth weights, she says these risks are extremely small, even when the recommended caffeine limits are exceeded.

Biostatistician Adam Jacobs has also pointed out the less rigorous nature of a narrative review and the bias that might seep in when analysing past results. Findings that do not show any link between caffeine and adverse birth outcomes may not get published at the same rate, he says, and even when they do, a null result does not usually receive the same level of publicity.

"Overall, I think if I were a pregnant woman, I don't think this paper would make me overly concerned about drinking the occasional cup of coffee," Jacob concluded.

With that in mind, it's important to address a newly published study that has called current guidelines into question.

In the new study, due to appear in the March 15 issue of Neuropharmacology, researchers scanned the brains of over 4,000 9- and 10-year-old kids who were exposed to caffeine more than once a week in utero. Their brains showed similar changes in certain brain pathways, and the authors say this was associated with minimal yet noticeable behavioural issues, including attention difficulties and hyperactivity.

The authors therefore conclude that "current guidelines regarding limiting caffeine intake during pregnancy may require some recalibration."

But one study isn't necessarily enough to influence these guidelines. Most data suggest a daily caffeine dose of 300 mg or less does not increase the risk of adverse pregnancy, fertility or neurodevelopmental outcomes.

Above this amount, the risks become more contested. A study in Norway found that pregnant women reported consuming only 130 mg a day on average, and usually this decreases as their pregnancy progresses.

The long-term outcomes of drinking caffeine more than once a week are also extremely difficult to tease apart from all the other environmental and genetic factors that could possibly influence behavioural and brain changes in developing children. Especially when parents were being asked to recall their caffeine intake ten years later.

In 2020, a study on caffeine consumption during 64,000 pregnancies found low to moderate doses of this drug showed no negative neurodevelopmental outcomes in children up to 8 years of age.

Research in 2012 on caffeine and behavioural changes in children found no reason for pregnant people to stop their consumption of tea, coffee or soda. Even the shorter-term effects of high levels of caffeine during pregnancy, like miscarriage, remain largely undetermined.

It's not that this new study on neurodevelopment should be dismissed; it just needs to be put into context.

Research on how caffeine can impact the developing brain is limited, and most studies exploring this subject have only found behavioural risks associated with high levels of caffeine intake. The new study offers an interesting biological pathway to explain some of those results, but how this translates to practical advice for pregnant people remains unclear.

Especially when the authors admit the exact dose of caffeine exposure throughout the current investigation is "highly uncertain".

"Current clinical guidelines already suggest limiting caffeine intake during pregnancy – no more than two normal cups of coffee a day," says author Zachary Christensen from the University of Rochester.

"In the long term, we hope to develop better guidance for mothers, but in the meantime, they should ask their doctor as concerns arise."

The study was published in Neuropharmacology.