‘No safe level’ of caffeine consumption for pregnant women and would-be mothers
Published 28 August 2020
A review of findings from observational studies and meta-analyses concludes that maternal caffeine consumption is ‘reliably’ associated with several adverse outcomes of pregnancy. Pregnant women advised ‘to avoid caffeine’.
Few everyday beverages have had such a roasting over the years as coffee. The target has usually been coronary heart disease but despite a plethora of studies there is still no consensus other than a vague agreement that two or three cups a day won’t damage the heart. Now, however, the coffee controversy has spilt over into the fertile ground of pregnancy, with a new warning that women who are pregnant or trying to conceive should be advised to avoid caffeine in the best interests of safe pregnancy and term delivery.(1)
Behind the advice lies a ‘narrative review’ of 48 original observational studies and meta-analyses published in the past two decades reporting results of coffee associations with one or more of six negative pregnancy and neonatal outcomes: miscarriage, stillbirth, low birth weight and/or small for gestational age, preterm birth, childhood acute leukaemia, and childhood overweight and obesity. The study’s aim, said the author, was to review current evidence on caffeine-related pregnancy outcomes ‘to determine whether the recommended safe level of consumption for pregnant women is soundly based’.
Although many observational studies have already reported possible increased risks for several negative pregnancy outcomes, current policy advice assumes that 'moderate' caffeine consumption during pregnancy appears to be safe. Many professional organisations, including the UK’s NHS, the American College of Obstetricians and Gynecologists, the European Food Safety Authority, set this safe level at 200 mg caffeine, which, says the author, approximates to two cups of moderate-strength coffee per day.
Now, however, the sole author of this monumental review (1261 papers initially identified), Professor Jack James of Reykjavik University, Iceland, actually concludes that ‘there is no safe level of consumption’. He adds that the dose-responsive nature of the associations between caffeine and adverse pregnancy outcomes found in his review - and the fact some studies found no threshold below which negative outcomes were absent - supports ‘likely causation rather than mere association’.
A total of 42 separate findings were reported from 37 observational studies; of these 32 found that caffeine was significantly associated with an increased risk of adverse pregnancy outcomes and ten found no or inconclusive associations. Caffeine-related risk was reported with moderate to high levels of consistency for all adverse pregnancy outcomes except preterm birth. In 14 of 17 meta-analyses assessed maternal caffeine consumption was associated with increased risk for four adverse outcomes: miscarriage, stillbirth, low birth weight and/or small for gestational age, and childhood leukaemia. The published report, in its summary of findings, describes these associations as ‘reliable’, and robust to threats from potential confounding and misclassification. The report thus concludes that the cumulative scientific evidence ‘supports advice to pregnant women and women contemplating pregnancy to avoid caffeine’.
As background to the study the report explains that caffeine consumed in pregnancy crosses the placenta and thus exposes the fetus to concentrations similar to those found in the mother – with effects evident in constriction of cerebral and coronary blood vessels, renal diuresis, respiratory bronchodilatation and gastrointestinal acid secretion.
This, while indrcating biological plausibility, does not prove causation, which observational data are rarely able to do. However, in this case the author notes that ‘likely causation is supported by a compelling body of evidence, both theoretical (ie, biological plausibility) and empirical, including a strong consensus among observational studies and particularly meta-analyses, dose–response relationships and reported absence of threshold effects’. Results, however, could be affected by other confounding factors, such as recall of coffee consumption, maternal cigarette smoking and most importantly pregnancy symptoms.
What also poses a problem in stirring the coffee controversy in pregnancy is estimating the burden of harm, as the author acknowledges. Many women, of course, will drink coffee during their pregnancies (even above the ‘safe’ thresholds) without any evident effect, but a calculation based on this review’s findings and US data shows that general consumption at this 'safe' limit ‘would account for approximately 350,000 negative pregnancy outcomes per year in the USA’ (about one-quarter of the population total attributable to the four outcomes of miscarriage, stillbirth, low birth weight and small-for-gestational age).
The reliance on observational data also raises the question of randomised trials in evaluating the evidence. One such trial reported only a moderate effect, but more generally the author notes the ethical paradox of randomising pregnant women to caffeine or no caffeine, when the former is here associated with harm, even if empirically. And he remains strong in his resolution that pregnant women and would-be mothers are best advised to avoid caffeine.
1. James JE. Maternal caffeine consumption and pregnancy outcomes: a narrative review with implications for advice to mothers and mothers-to-be. BMJ Evidence Based Medicine 2020: doi 10.1136/bmjebm-2020-1114324.