Having a baby is a big undertaking, and as such, it comes with a whole set of responsibilities and sacrifices. That in itself may not be surprising news – but what you might not have realized is that many of them actually begin well before your tiny half-clone ever sees the light of day. From the moment you see that extra line on the pregnancy test, your life is beholden to an ever-changing list of things you can’t do, things you shouldn’t do, and things you’ve never done before but are now told you must absolutely not forget to do on a daily basis.

Now, some of those prohibitions are easier than others – after all, there can’t be that many people out there living off an all-shark diet. But others can be more troublesome. Studies have suggested for years now that caffeine – that pick-me-up relied on by about four-fifths of Americans to get them awake and alert each morning – is not safe in any amount during pregnancy, with its consumption linked to higher rates of miscarriage, stillbirth, low birth weight, and childhood acute leukemia.

Compared to those outcomes, being a bit short might not seem so bad – but this is the result that has most recently made headlines. In a paper published just yesterday, researchers found that children exposed to caffeine and caffeine metabolites in the womb were up to 2 centimeters (0.8 inches) shorter at ages four and eight than their peers without such a history.

And we’re not talking about parents mainlining espresso day and night here: the correlation was evident “even with maternal consumption below current recommendations of 200 mg day,” the authors write. That’s the equivalent of about one cup of coffee all day – an amount which, under normal circumstances, would barely register, but which the researchers found was comparable to a smoking habit in terms of reduced height in childhood.

So, is it time to outlaw caffeine during pregnancy, thus avoiding a terrible future in which everybody is slightly less than an inch shorter than they might otherwise be? Well, let’s not be too hasty: 

“This type of study is somewhat controversial since it can only demonstrate an association, not causation,” pointed out Alex Polyakov, a Clinical Associate Professor at the Faculty of Medicine, Dentistry and Health Sciences at the University of Melbourne, and Consultant Obstetrician, Gynecologist and Fertility Specialist at Melbourne IVF and the Royal Women's Hospital, Melbourne, in a statement for the Australian Science Media Centre.

It's an inescapable problem with studies like these, but randomized trials, which may actually stand some chance of proving a causal effect, simply aren’t ethical when it comes to exposing babies to potentially hazardous substances. The best researchers can do is a retrospective study, where they compare outcomes from pre-existing datasets and try to find a link between them.

“The underlying assumption is that women who consume less coffee are identical in all respects to women who consume more, and authors go to great length to ensure that this is the case by using quite sophisticated statistical techniques,” Polyakov, who was not involved in the study, added. “Unfortunately, this ‘singular difference’ cannot ever be completely achieved.”

That’s why this particular study is seeing something of a pushback from some academics. “[The] conclusion that ‘increasing levels’ of caffeine and paraxanthine, even in low amounts, was associated with shorter stature in early childhood is incorrect,” commented Gavin Pereira, a Professor of Epidemiology and Biostatistics at Curtin University. 

“This conclusion indirectly implies that as their consumption of caffeine increases, risk of shorter stature also increases,” he explained. “Rather, all that can be concluded from this study is that the children born to women who consumed higher levels of caffeine were shorter than the children born to women who consumed relatively lower levels of caffeine.”

And there are many potential reasons why that might be the case. Perhaps the researchers are right, and it really is that caffeine exposure in the womb hinders growth later on in childhood – but then again, it may be that working longer hours during pregnancy leads a person to both drink more caffeine and also experience more stress. Perhaps it's stress levels during pregnancy, and not caffeine, which is responsible for the shorter height in their children. 

Alternatively, maybe having a higher caffeine intake is a symptom of having a poor diet; in that case, perhaps malnutrition is playing a part in the offspring's height after birth. Heck, maybe it's just that shorter people need more caffeine in the day to make up for all the extra energy we expend reaching up high for things that have been placed slightly out of reach – the point is the study did not, and indeed could not, account for any of these possibilities. We just don't know.

But that doesn’t mean the study should be disregarded completely. “We already know from previous research that there is a link between high caffeine consumption (>300mg a day) and lower birthweight and preterm birth, and intakes of more than 350mg a day are associated with pregnancy loss (miscarriage and stillbirth),” commented Helena Gibson-Moore, a nutrition scientist at the British Nutrition Foundation, in a statement for the UK’s Science Media Centre.

And while the study falls short of proving a causal relationship, Gibson-Moore pointed out that “the association between maternal caffeine consumption and decreased child height, is biologically plausible given that caffeine and paraxanthine cross the placenta.”

Essentially, this study probably shouldn’t send you into a panic if you’re currently or recently pregnant and partial to a cup of joe now and then – but it’s a useful reminder that caffeine, the most widely used psychoactive substance in the world, is something that doesn’t carry zero risk to the developing fetus.  

That said, there are very few things out there that do. Even paracetamol, sometimes known as acetaminophen, has been downgraded recently, moving from “one of the only painkillers available to pregnant people” to “might be linked to neurological disorders, language delays, decreased IQ, and more.”

Overall, Polyakov advised, “a healthy balanced diet is the best possible strategy.” A small amount of coffee during pregnancy, he said, is “rather unlikely to result in significant harm to either a pregnant person, or the offspring, notwithstanding the findings of this study” – and indeed, reading too far into the results of studies like this may actually do more harm than good. 

“Advice to unnecessarily restrict pregnant women’s diets may in fact be counterproductive and create anxiety and the feeling of guilt should women not be able to adhere to them,” Polyakov said. “The bottom line is that if one’s wellbeing is enhanced by having a cup of coffee, there is most likely no harm in having it."

The paper was published in JAMA Network Open.

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