EXPERT REACTION: Could a little caffeine during your pregnancy make your offspring short?

Consumption of coffee in pregnancy is an area of extensive research and there are multiple studies, which often produce conflicting results. Investigators have looked at coffee consumption and infertility risk/fertility treatment outcomes, risks of miscarriage, risk of gestational diabetes/preeclampsia/preterm labour, children’s weight/height/BMI, among many other outcomes.

This study is just the latest in a very long list of retrospective studies which assess the association of coffee consumption in pregnancy with offspring’s anthropometric outcomes, such as weight and height. This type of study is somewhat controversial since it can only demonstrate an association, not causation. 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.

Unfortunately, this “singular difference” cannot ever be completely achieved. It is possible, perhaps even likely, that those who drink more coffee may have other significant differences in their diet, lifestyle or some other unknown factor that would explain the findings and coffee consumption is not causally linked to the outcomes under study. Furthermore, when sophisticated statistical modelling is used and multiple outcomes are examined simultaneously, the possibility of finding a statistically significant association increases. The clinical or practical relevance of such findings is questionable. It is no coincidence that these types of studies are often referred to as a “fishing expedition”. If you fish long enough and use different fishing techniques, eventually you will catch a fish, even though it may be too small to feed you.

There are however some facts that are not controversial. Fact one: coffee has no nutritional value and therefore is not essential for a healthy pregnancy. Fact two: caffeine is present in low amounts in other beverages and foods, apart from coffee, such as tea and chocolate as well as many soft drinks, so complete avoidance is possible but may be difficult to achieve. Fact three: moderation is the key – it is probably true that excessive amounts of any product consumed during pregnancy may be associated with possible harms. This is true for coffee, but it is also true for meat and fish.

A healthy balanced diet is the best possible strategy and a small amount of coffee is rather unlikely to result in significant harm to either a pregnant person, or the offspring, notwithstanding the findings of this study. 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. 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.

There remains insufficient evidence that caffeine during pregnancy hampers the growth of their subsequently born children. The study by Gleason and others published in JAMA Network Open neither changes this conclusion nor contributes to the body of evidence on the topic. The researchers' attempt to control for confounding was severely limited to the little information that was available to them.

Their conclusion that "increasing levels" of caffeine and paraxanthine, even in low amounts, was associated with shorter stature in early childhood is incorrect. This conclusion indirectly implies that as their consumption of caffeine increases, risk of shorter stature also increases. 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. The correlation observed in this study can be explained by the existence of a common cause of both caffeine consumption and growth restriction e.g., poverty, stress, and dietary factors.

As a prenatal dietitian I find that many pregnant mums are confused by guidelines that suggest caffeine doses as they don’t understand what this translates to in terms of food and beverage consumption. This study found that even 50mg/day of caffeine may have a significant impact on children’s height. Fifty mg/day is equivalent to approximately one cup of tea or half a cup of instant coffee or a family-sized block of chocolate.

This study adds to the growing body of evidence that caffeine intake during pregnancy has a more significant impact than was once thought, and given there are no advantages for caffeine intake during pregnancy, it seems prudent to advise pregnant women to avoid caffeine intake if possible.