That fresh cup of coffee might be a neat pick-me-up, but for pregnant women it’s a downer, as far as science is concerned.
Drinking coffee during pregnancy has been linked with having fatter children, low birth weight, changes in a baby’s brain structure and subsequent behavioural issues, and risk of miscarriage.
And now, analysis from the US National Institute of Child Health and Human Development has found that even a small amount of coffee may lead to having shorter children.
The height difference became more pronounced over four years, from the ages of four to eight.
It’s unknown if these height differences are harmful.
How did they find this?
The researchers analysed blood samples from more than 2400 pregnant women in two previous studies, using concentrations of caffeine and its metabolite paraxanthine to determine how much coffee the women were drinking.
They then looked for correlations between caffeine consumption and child height, weight, body mass index (BMI) and obesity risk.
Child height was the only measure to show a consistent pattern in both study populations.
How much shorter?
Among 788 children in the Environmental Influences on Child Health Outcomes (ECHO) study, “those born to women who consumed the greatest amounts of caffeine were an average of 1.5 centimetres shorter than those whose mothers had the lowest caffeine intake”.
On average, pregnant women in this study consumed less than 50 mg/day caffeine.
Child growth was measured once, at an average age of seven years.
Children participating in the Collaborative Perinatal Project (CPP), conducted in the 1960s and 1970s, were measured at regular intervals from birth to age eight.
This allowed researchers to track changes in height over time.
Beginning at age four, children of women who consumed the highest amounts of caffeine were shorter than those born to women in the lowest caffeine-consumption group.
This gap increased from 0.68cm at age four to 2.2cm at age eight.
Caffeine consumption among pregnant CPP participants averaged around 200 mg/day.
Dr Katherine Grantz, one of the study leaders, said: “Our findings suggest that even low caffeine intake during pregnancy can have long-term effects on child growth.”
But she noted that the height differences observed were small “and further research is needed to determine if these differences have any effects on child health”.
Does the study hold up?
The main criticism of this and other studies looking at coffee consumption by pregnant women and the potential effects this may have on their unborn children is that a persuasive causal relationship hasn’t been established.
The Science Media Centre has posted some more detailed analysis from Australian experts.
Dr Alex Polyakov, a clinical associate professor at the Faculty of Medicine, Dentistry and Health Sciences, University of Melbourne and consultant obstetrician, gynaecologist and fertility specialist at Melbourne IVF and the Royal Women’s Hospital, Melbourne, said:
“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 lengths 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.”
Dr Polyakov advises that “a small amount of coffee is rather unlikely to result in significant harm to either a pregnant person or the offspring”.
He also notes that “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”.
Dr Gavin Pereira, a professor of epidemiology and biostatistics at Curtin University, said: “There remains insufficient evidence that caffeine during pregnancy hampers the growth of their subsequently born children.”
He said that “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”.
But this correlation “can be explained by the existence of a common cause of both caffeine consumption and growth restriction”.
These include poverty, stress and dietary factors.
Indeed, while the analysis took into account maternal height, pre-pregnancy weight and smoking status, it apparently didn’t look at diet or socio-economic background.
None of this means the study is wrong, it just hasn’t been proven.
Still, pregnant women are advised to be cautious
According to an explainer from the Royal Hospital for Women in Sydney, the recommended limit in Australia is a maximum of 200mg caffeine per day.
This would be equivalent to one cup of strong espresso-style coffee, three cups of instant coffee, four cups of medium-strength tea, four cups of cocoa or hot chocolate, or four cans of cola.
The Royal notes that due to limitations in scientific research, “it is difficult to determine whether a mother’s caffeine intake in pregnancy is associated with problems for the baby during and after pregnancy”.
What is known
Caffeine passes from mother to baby across the placenta during pregnancy and in breastmilk after birth.
Unborn babies are less able to break down caffeine than adults.
This means that babies may be exposed to the same stimulant effect as their mother.
Overall it appears that low to moderate caffeine intake does not make it harder to conceive a baby, nor does it increase the risk of miscarriage.
However, higher levels of caffeine intake (greater than 300mg per day) may be associated with decreasing the chances of becoming pregnant and increasing miscarriage rates.
The Royal advises that going caffeine-free during pregnancy may be the safest option.
