A total of 42 separate findings were reported; of these 32 found that caffeine significantly increased risk of adverse pregnancy outcomes and 10 found no or inconclusive associations.
Professor Jack James of Reykjavik University, Iceland, said: “Caffeine-related risk was reported with moderate to high levels of consistency for all pregnancy outcomes except preterm birth.”
Fourteen of the studies linked maternal caffeine consumption with increased risk of four adverse outcomes: miscarriage, stillbirth, low birth weight, and childhood acute leukaemia.
The studies were observational, meaning they could not prove that caffeine intake caused the risk. However, researchers said it was likely that caffeine was the cause, as the chance of negative outcomes increased depending on how much was drunk. No level of caffeine consumption was found to be free of risk.
Until now, pregnant women have been advised until now that consuming a small amount of caffeine daily will not harm their baby.
The NHS and the European Food Safety Authority (EFSA) both suggest that the safe limit is 200 mg caffeine, which is the equivalent to around two cups of moderate-strength coffee per day.
Professor James said such advice “is not consistent with the level of threat” shown by the research.
He said: “Current health recommendations concerning caffeine consumption during pregnancy are in need of radical revision. Specifically, the cumulative scientific evidence supports pregnant women and women contemplating pregnancy being advised to avoid caffeine.”
Caffeine is a stimulant and can increase blood pressure, heart rate and stress hormones.
A number of studies on caffeine have had contradictory results, with some research suggesting that more than 200g a day can double the risk of miscarriage, while other studies found that up to 300g was safe.
Scientists said women who drank a lot of caffeine might be more likely to have unhealthy habits, which might not be fully taken into account.
Dr Daghni Rajasingham, Consultant Obstetrician and spokesperson for the Royal College of Obstetricians and Gynaecologists, said: “The findings of this study add to the large body of evidence that supports limited caffeine intake during pregnancy, but pregnant women do not need to completely cut out caffeine, as this paper suggests.
“As the study notes, high levels of caffeine during pregnancy can lead to miscarriage and babies having a low birth weight, and may lead to excess weight gain in the child’s early years, which can increase risk of health problems later in life. However, as other – and potentially more reliable – research has found, pregnant women do not need to cut caffeine out entirely because these risks are extremely small, even if the recommended caffeine limits are exceeded.
“The Royal College of Obstetricians and Gynaecologists’ advise to limit caffeine intake to 200 milligrams (mg) per day – the equivalent to two cups of instant coffee – still stands. This paper does not supersede all the other evidence that has found that a limited intake of caffeine is safe for the majority of pregnant women.”
Dr Adam Jacobs, Associate Director of Biostatistics, Premier Research, said: “Given that pregnant women have been advised to avoid excessive caffeine consumption for at least the last 40 years, you might expect that women who drink coffee during pregnancy are generally less likely to follow health advice, and possibly in some ways which are quite hard to measure.
“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.”
Prof Andrew Shennan, Professor of Obstetrics, King’s College London, said it was possible that women were under-estimating their coffee consumption, and said it remained clear that high doses of caffeine did most harm.
“It is well known that women may report lower intake then actual consumption in studies and could explain why lower caffeine intake looks harmful,” he said.
“The authors conclude the advice from health bodies such as the NHS needs to radically change and they advise avoiding all caffeine. However the observational nature of this data with its inherent bias does not indicate with any certainty that low doses of caffeine are harmful, and the current advice to avoid high doses of caffeine are unlikely to change.”