HIGH-street coffee shops could pose a risk to pregnant women due to large variations in caffeine content, according to new research.
Analysis of espresso coffees from 20 shops found one with a caffeine content six times higher than another.
Researchers said customers were completely unaware of the caffeine levels in cups of coffee.
Health experts advise that pregnant women should restrict their average daily caffeine content to 200mg.
But while a cup of coffee from Starbucks was found to have 51mg of caffeine, a Costa coffee had 157mg.
The highest amount of caffeine found by the researchers was a cup containing 322mg of caffeine.
Experts warn that too much caffeine could result in miscarriage or a baby having a lower birth weight, which can increase the risk of some health conditions in later life.
Dr Alan Crozier, from the research team, said: "Despite the increasing number of coffee shops on the high street and in airports, there appears to be no recent publications on the caffeine contents of the various types of commercially prepared coffees.
"This snap-shot of high-street espresso coffees suggests the published assumption that a cup of strong coffee contains 50mg of caffeine may be misleading."
Dr Crozier added that the size of the coffee was the most important influence to the amount of caffeine.
He said: "The main factor is that some places use a lot more coffee than other outlets to make an espresso, which is fine for some people but pregnant women could unwittingly consume more caffeine than is good for them while thinking they are well within their recommended limit."